Well, here's a really far out guess...This may have been an initiator for a
scuttling charge...
Screw the gun into a spigot attached to a 55 gallon drum of blasting powder, on board a ship...Use an incendiary round...Shoot it into the drum full of powder to destroy your U-boat before it can fall into enemy hands...
Hey, that's as reasonable as some of the BS I read on GB...
(Feel free to use this explanation in your sales description)...