That's a copy of a Tubb CWS, but not a genuine one. The Tubb CWS tapers at the back, and doesn't have squared-off edges. Performs the same function, though, that piece you have. Weigh it, and see what it comes up at. Get a shot of the back of it, too. The real part adds 0.080" length to the overall of the BCG, so measure that back lip on the thing, and see how much it's adding... The CWS has three different weights that you can set it up as - just the body, body with steel insert, body with tungsten insert.