Inglis recoil springs.

[browningautorifle]

The high power ammo was for the SMG C1 not the stens. They would have been long out of use by the time we split our ammo into two types. We used the same ammo for years and then they came out with a higher powered ammo for the SMG, marked it "For SMG only"...but we used it back and forth some. Makes sense though, you asking for an Inglis spring only. I should think you can get one from Brownells rated at 22 lbs or so...should do...