Actually not near as rare as you might think.

A prewar Masterpiece with a micro cockeyed hammer sold a couple weeks back on GB.

Just this past winter I held a Colt revolver with Micro full rib on top. Their version was a lot clunkier looking and much heavier.

Sorry I have no pictures to share, but I might be able to get some pics of the ribbed gun if I the owner hasn't sold it yet.