Excellent info. 86,000+ mk3s.... and even the youngest is now 40+ years old.

That is a serious amount of potential springs that (based on current availability from 'modern' sources)...
...don't fit...
...don't work...
...don't match...
...don't belong....

My replacement original spring arrived from New Zealand, courtesy of the unbelievably generous Barry I.
webley tagged - Popped straight in - sweet as cherry pie.
Gun shoots smooth and has now settled down with no b@ggering about at 9.4 ft lbs (+/- 12fps and getting better).

The spring clearly suits the gun, and that suits me fine.