Since identifying the error with the Dragon 7.5cm gun housing and barrel length being 1.5mm too short in overall length,
I have investigated further to identify how such a fundamental error can happen in one kit but be correct in the other.

It all starts from the way the mantlet and gun housing parts are broken down in each kit, the DR kit has the inner trunnion and gun housing moulded in one piece with a separate outer mantlet.
On the TRI kit the mantlet and gun housing are separate parts with a further separate inner trunnion part.

After joining the TRI mantlet and gun housing together and then measuring the length of the gun housing extending from the TRI mantlet and the length of the gun housing from the DR trunnion they are the same at 8.5mm
(which is the correct length according to all the 1:35 plans referred to).

What the DR designers have forgotten is you then have to add the mantlet over the gun housing/trunnion thereby reducing the length of visible gun housing by 1.5 leaving just 6mm exposed, which in turn reduces the overall length of the housing/barrel out from the mantlet by the same 1.5mm.

As the TRI housing is attached directly to the mantlet the correct length of exposed housing and overall length is maintained when the mantlet is attached to the trunnion through the turret front plate, but not so with the DR assembly.

It should be a fairly simple matter for DR to redesign the part taking into account the extra 1.5mm for the mantlet if they so wish to do so, but is as rather a difficult job for the modeller to fix.

You will need to carefully cut the gun housing from the DR trunnion, then fill in the housing opening in the separate mantlet and butt join the housing to the front of the mantlet in the same way it has been done in the TRI kit.

Please note that the only information being put forward claiming these finding are incorrect are either from the Company or Company associated "advisors" or "reviewers". I would very much welcome input from anyone not associated with the Company or without any vested interest to actually measure their kit and advise the findings compared to the available plans, thank you.