This particular one has a few "upgrades" from previous versions. The Weaver rail is a solid addition but there's also an extra harness point on the design at the back where it would screw into the existing Battery Tray Door for added stability. On previous designs, the piece would lift up from the connection points under stress from the shoulder stock when it was in use. As such, the anchor point over the battery door will prevent that issue from coming up. The measurements for this feature weren't QUITE right, so we'll be adjusting the design appropriately. That's what these test units are for!