billt is correct the 21SF designation is for a "slimmer frame" than the standard 21, which I own, and affectionately call the the railroad tie with a grip.
Previous generations of "G" guns had no grip adjustments...it was as it is...
Now it seems they jumped on the M+P bandwagon, of which I also own,....and went small, med, large....
All is good, keep the price of 10mm in perspective if you plan on shooting alot.
If it's not an issue, I believe m25 also has a G20.