been going through the spares, i know this is something to do with the Mk1 Tallboy i had years ago,

what is it? internally threaded one end, recessed allen fitting the other

cheers

Looks like a linkage axle.
Has it got a locking collet that goes in the end?

nope, just those bits

Look here

https://www.santacruzbicycles.com/en-GB/bike/tallboy/1

at the bottom of the page, like stu said it looks like a linkage pivot axle collet.

I’ve just replaced all the bearings and pivots on mine. All of the pivot axles have an externally threaded portion on the outer end. That could be from a Heckler or a Blur.