Avoid confusing the 7.7x58 with the 7.92X57.
A gunsmith who collects Japanese rifles posted on another board of accidentally blowing up a 7.7 Jap rifle when he got a 7.92x57 cartridge mixed in with his jap ammo.
Luckily this happened when remote testing in a firing booth.
Some 7.7 chambers will accept the 7.92 (8mm) but the chamber neck is normally too tight so the neck grips the case neck hard enough to cause excessive pressures. A Jap rifle with very loose chamber might survive this sort of mix up, but I wouldn't count on it.
The German rifle looks to be a "Duffle Cut" bring back. Troops would cut trophy rifle's fore ends under the lower band so the disassembled rifle would fit in a duffle bag. They sometimes spliced the cut off piece back when they reassembled the rifle.
I'd leave the rifle as it is, unless you find the missing pieces packed away somewhere.
I've seen the type of revolver shown here in one of my old reference books, I think it is a fairly rare and possibly valuable antique from early cartridge days.
Not sure but it may be a "teat fire" that loads from the front of the cylinder. If so it should have only a small hole at the rear of the chamber where the teat holding the priming compound would fit.