Hi SG,
Nice find. I can see the U 94 photo and handrail in UiF 19.
There is also a handrail a bit lower down on U 553 (which has the slat intake on the outside of the tower) on page 10 of UiF 17. U 553 did have the machine gun mounted in a different photo.
I think that U 94 didn't have the handrail originally and it was fitted at some point - perhaps 1942 or so. It could be other slat boats other than U 94 and U 553 had these handrails in 1942 but we don't see them in some photos because the photos were taken prior to the handrails being fitted.
Some other VIICs such as U 415 did have a handrail but this was about three-quarters of the way up the bulwark rather than near the top.
Cheers,
Dougie