The G.M. in D.R.G.M. is "GebrauchsMuster" -- not as rigorous as a patent & only applied to articles of utility. I have come across a few GebrauchsMuster registrations corresponding to U.S. wrench patents. As noted above, the design is pretty much a copy of the Bullard patent with some slight tweak of details. Thanks for the name of the manufacturing company -- I might get lucky with a search using Google books & turn up an advertisement (I had that kind of luck with a MAUSER quick-adjust ad from the early 1920s.)