Thanks for that. Both magazines have functioned fine in my CZ 511 for many years until I changed to the 512. I am aware that a 581 mag may sometimes not lock in place on a 511 due to the top edge of the finger grip catching on the magwell. This was not the case with my 511. If this is the case a light filing of the top edge on both sides sorts that out, a 5 minute job.