Well yes, as Kevin.303 says, some sort of mount, but it slopes up at about 5 degrees, and it has 17 indentations on the top. The top of the 'rib' is dovetail shaped, and slightly narrower than the dovetail on the top of the receiver. The fit to the barrel is very nice, and if it's sweated on there is no trace of the solder around the edges.
It seems to be a very professional, carefully done, screw-up.