Too true my thoughts exactly. 50+ years in water and there will be very little left of them. I know older versions were more substantial than the modern ones but still.I have got experience of handling them - and of disassembling them for QC purposes!
Knowing exactly how they were / are constructed, I wouldn't be at all surprised if some of them were still viable. Tinplated steel, sealed by crimping then dipped in paint might well mean that there had been no moisture penetration so the black powder inside might be OK. The percussion caps might have degraded and become unstable though... I wouldn't play with them!