I have a NIW KAC RAS with the SAS lower half. It works exactly like the regular M4 RAS, but the lower rail has its own locking mechanism and locks solidly in place. The SAS requested this feature to provide a more solid mounting arrangement on their M4s for the HK grenade launcher or a vertical foregrip. I have several of these on my own uppers and they work great.
I have this one left over from a build that never happened. It does not have any rail covers, but is totally new in wrapper. $250 shipped in lower 48, so you could get KAC rail covers and still be less $$ than retail for a regular RAS, or have the option of getting TangoDown covers and saving even more.
thanks for looking.
If interested, send an email to simmsNOSPAM@isot.com (just remove NOSPAM from the address).