Hammer Gun Background

[OH Osthaus]

this is not a Parker Brothers gun

I cannot see the proofs on the water table well enough to tell you British or Belgium

but unless there is a key slid in from the side - it seems to be a snap on forend - it may be tight (looks like the tip insert was lost to pulling on it at some point)

you could try a long slope wedge of wood - and gently tap it in from the muzzle end

if by any chance you have one of those old wooden spring action clothes pins- a half of that is a good tool