Since my post in January I've been looking at spools and cameras. It's very obvious with the Barnacks, FED-1's and Zorki-1's (and probably the very rare Zorki-2) that some take up spools will fit and some won't. So I was able to create two heaps of spools quickly and I have carefully measured 10 of them.
These are what fit the Barnacks, FED-1 and Zorki-1:-
The one on the left is a typical Leica spool; points to note are the criss-crossed knurled end and the arrow pointing to the clip. Not all of them have the arrow, btw. They all seem to be brass and some are painted and some blackened chemically. (The camera bodies checked date from 1926 to 1946.)
In the middle is a typical FED or Zenit or Zorki spool. It hasn't got the knurling like the Leica's and no arrow either but the clip is more or less the same. These and Leica ones sometimes have rounded corners to the clip.
Lastly, the one on the right with the nasty hook is from the old USSR and fits the Leicas etc but it's the exception. I had just two of them that fitted and dozens of hoooked ones that don't fit Barnacks etc.
As for measuring them; the Leica ones are about 0.024" shorter than the old USSR's ones. The internal diametres are the same but the clue as to their fit seems to be the diametre of the take up spools' slipping clutch post and its length. The ones that don't fit Barnacks etc all stick out too much for the bottom plate to close.
So the visual clues (for buyers) seem to be the clip's shape, knurling and sometimes the arrow. BTW, one of these Barnack fitting spools came in a Zenit from the 1950's.
I hope this is of some use. I wish sellers on ebay and so on would read this, they all think a spool will fit (say) a FED-1 and
a FED-2 and so on and vice versa but the 1's are the exception. One of these days I'll start sending spools back...