Colt never offered a real 20 rd. mag for the 7.62x39, R068XX series AR's!! They only offered a 5 rounder that was based upon the 20 rd. standard AR mag. body with a special riveted in insert that limited capacity to 5 rds. The floor plate was stamped with the 7 .62x39 caliber designation. If you remove the rivet/insert.....you can load up to 8 rds.......but the cartridges will not feed reliably......only with 5 rds or less!
I have some new 30 rd. "Frankenmags" for sale if interested......they work very well.....in even the M-16's with an 7.62x39 upper conversion!! E-Mail : firstname.lastname@example.org if interested.....?