Now you see, that's the trouble with these threads that go on forever---we forget stuff. Post #42 provides all the information "to gauge how big that spring is"---never mind the spring pictured is almost certainly an original, and that in post #42 is likewise "almost certainly" simply one that works.

It would be a relatively small task to confirm that one way or the other, but I have to go make short grass. So I'll leave you with this little tid-bit (which you already know): Any spring which will fit over the post on the cam and into the hole in the barrel will do just fine---after some scientific fitting (known as "cut and try")--and then maybe cut some more---or say a few bad words, and start over with a new (and longer) spring.

Ralph Tremaine

And Guy's probably right about a rebound slide spring. It wouldn't fit very well, but it'd likely work.