I thought I’d share this in the hope of preventing others’ problems.
Recently my plunger A10 return spring broke. I ordered another from a well known supplier and fitted it. Took a lot of effort to tighten up the gearbox cover which cracked! On inspection of the old and new springs the old one had a hook to fit the quadrant of about 3/8” long and flattened. The new one was over an inch long and round along it’s length. It prevents the cover seating properly.
Now the cover is repaired (beautiful, invisible job) and a suitably altered spring has been fitted.
Just wanted to warn anyone likely to come across this to make sure the new spring fits properly.