Standard G20/21 mags use the same spring (#448) in Gen 1/2/3 magazines.
Current SF mags (and a few gen 3) use a special spring called the wide base (#1300) designed for use with 1304 (#1,2,3 followers).
I discussed this issue with the KRISS crew. They informed me the only requirement was a new 10mm follower.
If they are aware of the difference, this means I do not have to make one. I guess I am content to wait for them to develop it for me.