definately an after market addition Stewart, it depends on how it is attatched to how much a DIY job it is (with screws, that needed the body to be taped with a thread) and just how handy you grandad was, whether it was done by him or a camera shop or someone else...or whether he was perhaps the second owner...the wierd thing is, is why is it mounted sideways..........😕
my only other thought is for some kind of flash atatchment that was conected via the shutter cable socket (some crazy looking flash setups available back then), although i still would of thought it should have been put on frontways! maybe someone goofed and it was never used..... or they felt they had to mount it that way to avoid the screws impeading on the back door...