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Apparently a MKII Bren recoil spring once sold at Princess Auto for a couple bucks fit fine and delivered 600+fps. As well HW30/R7 ,Diana 27 and BSA Meteor spring fit.Not confirmed.Harold
I actually have a couple of them. The PA springs, that is.
I put one in to a very tired (could not unseat a pellet from the breech when fired) Relum that I had apart, and it still shoots pretty well. Was two dollars well spent!
Again with the CTire caliper, as knowing the inside and outside diameter. As well as the wire size and coil count, will give you a better than average chance of finding a suitable substitute to use.