I've had this same problem in 25 years of fixing old airguns, mostly long-obsolete models. Jim Maccari aka 'the spring man' in the U.S. has been a great resource for springs but he doesn't list all the dimensions of his offerings. Just today I found that the Vortek website is offering a variety of springs with all dimensions shown (Vortek's Tom Gore is best known for gas-spring kits to suit modern guns):
http://vortekproducts.com/ourstore/a...ir-Gun-Springs

I've bought used springs off _bay and out of odds-boxes at airgun shows which have been useful for old guns, and likely some OEM springs from current or recent guns will suit our vintage guns. But it would be really convenient to have original spring dimensions for various models so we'd know a) what the original specs were in the old guns, and b) what current springs would work as replacements.

As to "old vs modern metallurgy", this is really not an issue: higher-strength springs with better manufacturing methods simply allow for running closer to the stress limit of the material without early fatigue failure. A spring's RATE (force/deflection) depends ONLY on dimensions, as the elastic modulus of all types of steel is virtually identical.

Don R.