A customer is sending me an M21 fitted with the Kuhn selectable break circuit. Kuhn was a watchmaker here in Baltimore and I personally have been looking for one for years. He did very nice work. I saw 4 in one place but they were sold with the estate lot to a single buyer. Anyway, he is sending it to me for service and I told him I would do some research on the patent. From what I know, there are fewer than a dozen known. These were all done post WWII production prior to the 1960s. Now you know all I know. I think U.S. Coast and Geodetic Survey commissioned them. This was after market work and there would be no Hamilton records on them that I know. Does anyone have any info on this? Thanks.