There are a full 26 Models of the Immortal D-7 Battlecruiser, the only such ship class with that distinction. To alay confusion, here is information on all of the variants.
D-7A (K't'agga):
Launched in 2247, this was the first version of the D-7, with a quartet of Disruptors as the only armament, although sometimes Target Drones launched from one of it's two 7-drone racks could be fitted with Atomic Warheads and used as weapons, although they were slow, slightly less damaging than the weakest Accelerator Cannons or Photon Torpedoes, and until drone improvements could be made in 2268, could only be launched at Impulse. This ship was mass-produced, and the most common version of the D-7 before 2270.
D-7B (Klolode):
Launched in 2249, this was one of the first ship classes given a larger armament of nine Phasers, four Disruptors, and routinely upgraded Drone Racks, to have ships with massively overwhelming firepower, and while such ships were good but not great, and would only be an occasional sight over the next century and a half, the D-7Bs were highly successful, with some of them operating as flagships for squadrons and fleets.
D-7C (K't'kara):
Launched in 2254, these ships had Warp Engines switched to a lighter model, capable of slightly higher speeds, as well as additional Disruptors fore and aft, easily making for the cheapest, most accessible combat variant, due to the older, more readily available parts.
D-7D:
A Drone Battlecruiser, which, like the D-6D, was a variant with at least 2 more drone racks, but unlike the D-6D, was to simply have a generally better Cruiser.
D-7E:
Despite already having a decent smaller survey cruiser (the D-9B) for the long-range exploration missions, a few D-6Es and D-7Es were produced in 2262. They were mediocre, not particularly interesting, went down some in combat capability, and relied disconcertingly on Federation Sensors, Helm and Lab Technology.
D-7F:
A ship used for somewhat more intense, Intelligence, Medium-Range Exploratory Missions, Recon, and launched in 2263, they had stronger concentrations of Internal Security and better Assault Shuttles, using the same Phasers, Disruptors and Drones, although various Merculite, Neutronic, Antimatter and Haridain missiles were expperimented with.
D-7G (K'talla):
In 2267, with the Treaty of Smarba, 24 RPL-1 Plasma Torpedoes, of an older, low accuracy design, were traded to the Klingons, with 8 being sent to various research and development organizations, and the other 16 being fitted to D-7As, making the D-7G, and while they were fairly average Battlecruisers, especially having nothing different other than the improved Plasma Torpedo, Klingon Crews did not like having to constantly get the ships up to point blank to the target, which was dangerous, so even though Plasma would sometimes be fitted to Klingon Ships, apathy and poor battle reports led to no one bothering to developing anything stronger than an RPL-1, and only producing a few dozen RPL-1s a year from low-priority shipyards, one of which was producing K-17s and D-6s for backup Scouts and Light Cruiser/Destroyer type ships.
D-7H (Anarchist):
This was the designation given to the Hydran Refitted D-7. Obviously, Klingons did not use these ships, although they did put Hellbores on the QuD, Vod'leH, and one or two other Classes, to use quite successfully to down shields quickly. The HMS Anarchist herself served from 2285-2320, before being put in some kind of reserve duty/surplus depot with some limited museum tours.
D-7I:
A refit of the D-7F that came online in 2265, with improved internal security, brigs, sensors, and a few additional briefing rooms, a cheap, but effective intelligence ship.
D-7J:
A ship that experimented with Hellbores, mounting a single HHB-1 forward, and that mounted stronger phasers, a further attempt at mounting larger firepower on Klingon ships.
D-7K:
Some of the Phasers of older D-7Bs were replaced with improved KH-3s, and the D-7Ks also had minor shield improvements, which helped get some more mileage out of the Phaser/Disruptor/Drone ships.
D-7L (Demonslayer):
A slightly more powerful leader of D-7 Squadrons, with improved command facilities. Ships of this class had improved auxiliary reactors, and made further use of the Phaser/Disruptor/Drone Arsenal, and one ship, IKS Darkslayer, disappeared in 2281, later found to be kidnapped by the Masters and forced to play in their Arena, soon being modified to be similar to a D-7M with more shuttlecraft and transporters, carrying a CLoaking Device.
D-7M (K'tinga):
The first major refit of the D-7 for over a decade, in 2270, the D-7M was given more powerful Engines, KD-8 Disruptors, KD-6 Drones, and fore and aft KP-3 Torpedoes. Both this and the the D-7A have been produced at over a thousand each, and one ship, the IKS Bardur, has become renowned for destroying 20 Federation ships, 10 Romulan hips, and 10 ships of various other races, and her captain is an Admiral, the ship operating near a border station with a fleet of 25 ships as part of an Orion Pirate Countermeasures group.
D-7N:
A diplomatic refit Battlecruiser with Phasers and Disruptors, wiht a single Fighter carried for defense. this ship became the primary diplomatic transport of the Klingon Imperial Navy.
D-7O:
A Battlecruiser refitted with an experimental Stasis Field Generator, otherwise being similar to a standard D-7A. The Stasis Field generator was highly effective at immobilizing ships, but drained massive amounts of power, leading to one ship in a squadron being the sfg-equipped ship, and the other two ships being the combat and troop ships, with several variants and new classes being equipped with Stasis Field generators and serving in the General War.
D-7P:
A slightly more durable battlecruiser armed with both Photon and Plasma Torpedoes, Phasers and Disruptors, the D-7Ps were difficult to maintain, and expensive, so only a few were built.
D-7Q:
D-7B fitted with Batteries and a Mauler, and while such a ship was massively damaging, it was the muscle in a battle group, and while it was fairly successful, there weren't more than 20 or 30 built.
D-7R (K't'rika):
Another experiment with Plasma Torpedoes, this one using the improved D-7M Hull, just like the D-7S, it had a coulpe of improved Disruptors. This one was somewhat more successful than the D-7G.
D-7S (K't'mara):
A further upgraded D-7, with a pair of additional Disruptors, the D-7S used fore and aft KP-4 Torpedoes, and had a slightly more durable hull.
D-7T:
Refit of the D-7M developed to get more use out of the aging 23rd century battlecruisers with improved systems. These ships were lighter fleet support and light cruiser/destroyer substitutes.
D-7U:
Refit of the D-7T with the forward Torpedo Launcher replaced by a Heavy Disruptor Cannon, making for a further improved fire support unit and escort.
D-7V:
Carrier Refit of some of the final D-7Bs and D-7Ks, with a bigger Shuttle/Drone Module and more deck crews. These, with some modified assault ships and D-18 Destroyers, made for some particularly deadly assault groups.
D-7W:
An attempt to refit the D-7S with some 2Xe technology. Also carrying both Photon and Plasma Torpedoes, the vessels could slug it out with vessels somewhat bigger than them. One of them became Qo'nos'Wa, the Chanceller's Flagship.
D-7X:
Furthering the experiments with 1X, 2Xe and 2Xs Technology.
D-7Y:
Another Carrier variant, this time keeping most of the D-7K Weaponry and most of the fighters.
D-7Z:
D-7M converted with a Mauler, and numerous Battery Groups, there were only a few D-7Zs, but they were quite combat capable, like the D-7R, being an improvement with the improved KD-8s and Engines of the D-7M.