The ribs on the side of 6.5 Grendel mags are different than on 5.56/223 mags but the footprint is the same. The C Products Defense (CPD) Duramag 20 round and 28 round mags have a curvature more similar to 7.62x39 mags. The rib depth and location are there to keep the cartridges correctly stacked in the mag body. The ribs are different on the 6.5 Grendel mags because of the differences in the cartridge dimensions between 5.56 and 6.5 Grendel. Some pictures for comparison.
5.56/223 mag picture L-R Brownell’s curved 20 round, Brownell’s straight 20 round, Colt AWB ban era 9 round.
6.5 Grendel mags L-R E-Lander 17 round, C Products/ASC 15 round,E-Lander ten round, C Products/ASC ten round.
6.5 Grendel mags L-R, 17 round E-Lander, 10 round ASC, 20 round CPD Duramag.
Y
6.5 Grendel mags, Left is a 25 round ASC and Right is a 20 round CPD Duramag.