John wrote:

The cable looked like it was simply wrapped over itself three or four
times and then covered with some kind of heat shrinkable metal sleeve.
This metal sleeve was definitely shrunken over cable and you could see
that the bitter end of the cable was wrapped around the standing end.
This sleeve was not rubber or plastic like the heat shrink used in
wiring. I scraped at with my fingernail and it was definitely metal of
some sort. I would estimate now that the sleeve was at least two
inches long and maybe longer.

I've never heard or seen of such a thing, and I worked as a sailboat
rigger, and made aircraft cables on contract at the plant I worked at.

All I can think of is that this is a copper, dead soft aluminum, or
lead sleeve that's been smacked between the jaws of a punch press or
rotary swager, with RUBBER dies, to make the soft metal flow, conform,
and essentially room temperature forge around the cable.