Black Dog Machine’s .22 50 round magazine was launched late last year. It works with a variety of .22 Long Rifle AR-15 conversion kits and AR-style .22 rifles (Atchisson/Ciener style drop in conversion kits, Kel-Tec, Sig 522, Tactical Solutions uppers, Model 1 Sales and Spikes kits.). Price is $80.
The improvements in the new gen-3 mag are: steel feed lips, no need for magazine loader (follower can be manually pulled down) and 30-round capacity (old mags were 26-round capacity.)
[ Many thanks to Gregory for the info. ]