just to add to my previous post, there is a Knights RAS in the classified section right now for $125 op.
https://www.okshooters.com/showthread.php?181809-FS-KAC-URX-amp-M4-RAS-rails-and-and-EoTech-553
https://www.okshooters.com/showthread.php?181809-FS-KAC-URX-amp-M4-RAS-rails-and-and-EoTech-553