As said they are a short thinner wire that is attached to the end of the main wires before tithe battery designed to blow/melt (fusible link wise)
Probably not many Cosworth’s left with the original fusible links on them as like yours the insulation has dried and crumbled.
I’m not sure what is the correct thing to replace them with and don’t know what the rating is as maybe you could just cut them off & replace them with a normal inline fuse & holder of the appropriate rating, I was lucky enough to get som original ones from ford when they was still in stock.