At the beginning of chapter 17 of Adversus Marcionem, Book 5, Tertullian introduces Marcion's version of Ephesians with these words:
We have it on the true tradition of the Church, that this epistle was sent to the Ephesians, not to the Laodiceans. Marcion, however, was very desirous of giving it the new title (of Laodicean), as if he were extremely accurate in investigating such a point. But of what consequence are the titles, since in writing to a certain church the apostle did in fact write to all? It is certain that, whoever they were to whom he wrote, he declared Him to be God in Christ with whom all things agree which are predicted.
According to Tertullian, Marcion believed that what we know as Ephesians should be called Laodiceans. However, Tertullian dismisses this idea, and continues by commenting on what he believes is Ephesians, as covered on the Ephesians page. In contrast, Epiphanius comments on Ephesians in his copy of the Apostolicon without any suggestion that Marcion called it Laodiceans, but then at the end of his discussion of the epistles he writes:
This concludes Marcion’s arrangement <of the> remains of the words and their subject which he preserves from Luke’s Gospel and The Apostle. From it I have selected the parts of the material he retains which are against him, and have placed the refutations next to them. But in his own Apostolic Canon, as he called it, he also added, of the so-called Epistle to the Laodiceans:
“(There is) one Lord, one faith, one baptism, one God and Father of all, who is above all, and through all, and in all.” (Scholion 1 and 40)
In agreement with the Epistle to the Ephesians, Marcion, you have also gathered these testimonies against yourself from the so-called Epistle to the Laodiceans. Thus, at the end of the work, we may find what you have to say by reading it and, by finding what your teachings are, see through [Epiphanius here plays on ἀναγνόντες, γνόντες and καταγνῶσιν.] your heretical inventions, the three first principles with no first principles of their own which are different from each other. For the holy apostle’s thesis and his authentic preaching are nothing like this, but are different from your fabrication. He plainly meant, “(There is) one Lord, one faith, one baptism, one God, the same Father of all, the same above all, the same through all and in all’’— through the Law and the prophets, and in all the apostles and the rest. (Elenchus 1 and 40)
From the above it appears that Tertullian and Epiphanius saw very different versions of the Apostolicon: Tertullian saw a version of Ephesians that Marcion called Laodiceans, while Epiphanius saw Ephesians as Ephesians, and at the end of Marcion's "Apostolic Canon" he saw an additional epistle that was the "so-called Epistle to the Laodiceans," but contained at least some of the text of Ephesians 4:
There is one body, and one Spirit, even as ye are called in one hope of your calling; [4:4] One Lord, one faith, one baptism, [4:5] One God and Father of all, who is above all, and through all, and in you all. [4:6]
Nathanial Lardner comments on the different testimonies of Tertullian and Epiphanius:
There is no doubt that Epiphanius refers to Ephesians and Laodiceans as separate entities, and others have noted that an epistle named Laodiceans did exist at that time. Lardner notes that an: “epistle to the Laodiceans … is now extant under this name.” but does not contain the text reported by Epiphanius. Early Christian Writings notes that the Muratorian Fragment, after identifying the Gospels, Acts, and the thirteen Pauline Epistles (excluding Heb), states the following:
Moreover there is in circulation an epistle to the Laodiceans, and another to the Alexandrians, forged under the name of Paul, bearing on the heresy of Marcion, and several others, which cannot be received into the Catholic church, for gall ought not to be mingled with honey.
In ‘New Testament Apocrypha, Vol. 2: Writings Relating to the Apostles Apocalypses and Related Subjects’ Wilhelm Schneemelcher provides details of the ‘forgery’ referred to above:
This Latin Epistle to the Laodiceans is found in many Bible manuscripts, and was evidently widely disseminated in the West. There was also a series of translations into western vernaculars… But so far no evidence has been found of a Greek text. On the other hand, later Greek sources speak of an epistle to the Laodiceans (c.f. the compilation in Pink, op. cit.), so that we must at least assume that the existence of such an epistle was known in the East. The epistle probably came into being in the West…
The dating of the Epistle to the Laodiceans is difficult for the reason that it depends on the question of the identity of this apocryphon with the one mentioned in the Muratori Canon, and this again is closely connected with the problem of its Marcionite derivation. Either the Muratori Canon means the Epistle to the Ephesians, the name of which was changed by Marcion into the Epistle to the Laodiceans (so Tertullian) - that, however, is unlikely, since Ephesians is mentioned in the Muratori Canon - or it had actually in view a separate Epistle to the Laodiceans, and then it must be the Latin Epistle to the Laodiceans that has come down to us, if we are not to assume several pseudo-Pauline letters to Laodicea. Certainly the Latin Epistle to the Laodiceans shows no sort of Marcionite character such as ought to be expected according to the statement of the Muratori Canon…
To sum up, it may be said that the Marcionite origin of the Latin Epistle to the Laodiceans is an hypothesis that can neither be proved nor sustained. It is rather a clumsy forgery, the purpose of which is to have in the Pauline corpus the Epistle to the Laodiceans mentioned in Col. 4:16. Whether the Epistle to the Laodiceans mentioned in the Muratori Canon is identical with this apocryphon remains unsettled. With that possibility of an accurate dating also falls out. As the time of composition there comes into question the period between the 2nd century and the 4th.
Summary
The evidence leads to one conclusion: that Epiphanius saw in his copy of the Apostolicon both a version of Ephesians (named as such) and an epistle named Laodiceans that appears to be largely a ‘pastiche,’ and may have possibly been written for the express purpose of writing a letter that could be claimed to be the letter referred to in Colossians 4:16:
Salute the brethren which are in Laodicea, and Nymphas, and the church which is in his house. [Col 4:15] And when this epistle is read among you, cause that it be read also in the church of the Laodiceans; and that ye likewise read the epistle from Laodicea. [Col 4:16]
This idea is taken up in Paul’s Letter to the Laodiceans - Is it Lost? in the website ‘The Expository Files’:
Many have argued that the letter to the Laodiceans is the Epistle to the Ephesians. This view is very attractive for a number of reasons. The idea is that Ephesians is a circular letter, making the rounds of all the churches in the area. Following this line of argument invariably leads to discussion as to when Ephesians and Colossians were written but that cannot be conclusively resolved so as to exclude Paul from referring to the Ephesian letter in Colossians 4:16. Circular letters were used in New Testament times (see Revelation 1-3) and while the Scriptures do not explicitly say Ephesians was for all the churches in that area it is not impossible. This may be the solution to the question “Where is the letter to the Laodiceans?” It may well be in your Bible under the title “Ephesians!”
To the final point, the same letter may well have also been in Tertullian’s copy of the Apostolicon under the title “Laodiceans.”