Hi Simon,'
Makes sense. The Gallery hatch is part of the original build and so would have common fittings, whereas the various turms seem to be local brews based on a general set of plans so the hatch in view could have been retained with an adhoc mechanism, if not unique it could have been used either by one shipyard or even one construction crew.
This custom building is more noticeable with the type IXs where each boat seems to have a unique detail on it, perhaps this is so due to there being less of then, but I believe that the same diversity of fit and finish exists on the type VIIs, it's just with the numbers built it is harder to spot. I suspect that the Captains had a certain amount of say in how the turm was fitted out during upgrades, along with material availability gives each boat a good change of being unique,
Speaking of such things your latest post with the ammo container reminds me of of the diversity of the number and locations of the ready containers, (ammo and life raft).
Regards
Jon