The AX AICS is their pistol grip chassis. The rifle cut off on top of that poster is an AX or AX chassis.

Judging by that poster:
6616BL = 10rd 223 mag for AICS
3901 = 5rd 308 mag for AICS
3902 = 10rd 308 mag for AICS

I honestly didn't know they made that many mags, but it doesn't shock me. I'm American, so I have this crazy notion that we should try to standardize things and not over-complicate them to death.

I look forward to Magpul releasing their 5 round plastic AICS mag. It should rattle less! But really I only care because I'm expecting it to be under $30 street price.