I just pulled them all out of the safe, and five have the "improved lock" and two the "standard lock." All five with the "improved lock" have concave ribs, while the two with the "standard lock have the high flat ribs. Both guns with the "standard lock" have the raised safety slide, while the two earliest "improved lock" guns also have the raised safety slide. The late 1906, the 1907, and the 1909 guns have the dished out safety slide.