It is a .300 Blk 3-gill hexagonal design that I picked up somewhere in my e-shopping travels. I don't recall what brand or where, sorry.
BTW, the M10X Brake was a total PITA to remove! There are no wrench flats and I broke a tine off of the brake using a big-arsed screwdriver to try to lever it off by wedging the blade in the FH tines. I had to use a dremel with a cut-off wheel to cut through to the threads on 2 sides and snap the brake into two halves. I had to be very careful not to hit the barrel itself...
Hmmm wonder if it's just thread locked in or what. Glad I like the oem brake