Looks like a MX10 rib, but that's the first time I've seen the relieved area on the fixed area. The early ones I've seen were made of steel and the latch stuck out slightly on the sides. MX9's have a fixed rib, but the middle bead could be changed out because it was mounted to a removable wedge. It came with a numer of these wedges of varying height allowing one to keep the site picture they desired when raising lowering the comb, like am adjustable rib. There were a number of them floating around in the mid 90's and a few owned here in NE Ohio. Drew Waller used to shoot one quite often.