As I am sure that you know. The grams spring and follower is the best way that "I" know to have a sot at 21 rounds in the mag. I do know people that have 22 rounders. But it does take an other tube than SV or St tubes... Again that I know of. The ones that I have touched are not S_I's...
I also know that tube to tube is the biggest variable that will determine if you are going to get 19,20,21 in that bad boy. Dementionally that is. Inside width and overall length being consistent and correct.Some say that you need the grams base pad... But I have fond that though I do have a higher success rate with them, I can still make a dawson work. If the tube is just right. The tubes that I have had brownells guts in. I have never made 21 with that combo, or a good re loadable 20. My 20's with the later have been very tight ones at that.
Hope that is even close to what you might have been looking for.