The cap tubes of the 3rd generation are categorized into (at least) 3 versions(1) by the groove and vent hole position (Figure, Graph). Version 3.0 with two pairs of vent holes seems to be the earliest type. After version 3.0, the caps tube has a groove to equalize the pressure of inner cap section and the grip section. Version 3.1 has the same vent hole positions as version 3.0. In version 3.2, vent holes are located at the lower position than version 3.1 by 2 mm. In version 4, lower vent hole is not seen anymore. If my observation is correct, cap tubes might be developed and fitted to the pens in this order (there might be some exceptions*).
Figure. Cap top of Pelikan 100.
Graph
Tube ver.(1) Groove 1st hole(2) 2nd hole(2) Note(3) n(4)
3.0 - 5.3-5.5 mm 11.3-11.5 mm 3rd gen.(barrel w/o ring, barrel w/ ring) 4
3.1 + 5.2-5.4 mm 11.4-11.5 mm 3rd gen.(barrel w/ ring), 4th gen.(#110)* 3
3.2 + 7.21 mm 14.2 mm 3rd gen.(barrel w/ ring) 1
4 + 7.2-7.3 mm N/A 4th gen.(#100, #101, #111, #T111, #112) 8
Note; (1) “version” is not official and is for the descriptive purpose.
(2) Length from top to the center of the vent hole (mm).
(3) My pen(s) that have the cap tube in question. For some tubes, I have them only as parts. So, other combinations may be possible.
(4) Sample number.