the plates were made for the reciever... the key hole inletting insures that...
the fore arm, barrels & reciever are all matcing numbers... since the screw heads are also engraved, I don't think they have ever been removed... so I'm really hesitant to do it...
I'll paint a pic & outline the keyholing, since I must not be explaining it correctly...
the red outline is the rough meeting surface of the locks & the reciever ( sorry, crudely done quickly in paint ) hope this helps explain what I mean... this is something my local builder has never seen before... but he's admittedly a rifle guy, & not so much a shotgun guy...