The 221 Fireball and the 6.5 Grendel/7.62X39 are the only 527 magazines that I haven't converted to three rounders. I haven't owned a 527 in 221 and the three round 6.5/7.62 magazine would still extend about 1/2 inch below the bottom metal. I think you could make a 2 round 6.5/7.62 magazine that would sit flush but haven't tried it yet.
