Looks like the seal was made from a stuff called 'rubberoid'. That's exactly how it perished - into a sort of rubbery goo and impossible to get off your fingers. Used to find it on old .38 ammo boxes that'd been in store for ages.
I don't dispute it Son but wonder how a thick leather strap and buckle with widely spaced holes could safely hold the bolts. They'd be rolling about like a bulldogs balls before long. Maybe they were wrapped in that thick waxed paper.
Looking at the geometry of the box layout as shown and based on nothing more than that, it SEEMS to show that two rifles fit in there BUT the left hand rifle will be muzzle forwards and other rifle vice verca. Surely the bolts fthen ace INWARDS. Or am I missing sumfink?