Hi cap fuses class J 600 fuses.

http://www.idc-online.com/technical_...ering/Fuse.pdf
This is all i could find.

I picked up and absent mindly unscrewed part of a pole fuse few weeks ago, and it fell apart in my hands and a small 1 inch long peice of grey soft metal fell out. It looked like the same as a peice of large dia rosin cored Solder, but not as soft.
In wondered if it was Silver and how much was left inside the fuse.
Do fuses often have Silver in them?

This

And a quick wiki search tells me 'Yes'..
Quote wiki { Construction
The fuse elements used in most distribution cutouts are tin or silver alloy wires that melt when subjected to high enough current. Ampere ratings of fuse elements vary from 1 ampere to 200 amperes.}
https://en.m.wikipedia.org/wiki/Fuse_cutout