In the 1960's, with the success of the of the Seamaster 300, 3rd generation divers, the 165.024 and 166.024, Omega began their series of "baby" divers, the SM120s. Cheaper, Smaller and lighter, 37mm as opposed to the 41mm SM300, the SM120s were initially produced in 4 references. Unlike the SM300 which were only automatic, manual movements were offered as well. 31mm ladies sizes too, (I don't have any of these but be careful as some sellers neglect to mention they are selling the tiny size!!) THE FOUR REFERENCES: 135.027 Manual Movement, No Date, Caliber 601, 611 136.027 Manual Movement, with Date, Caliber 613 165.027 Auto Movement, No Date, Caliber 552 166.027 Auto Movement, with Date, Cailber 565 Below are pics of my SM120's. Most I think are fairly nice EXCEPT THE LAST EXAMPLE WHICH IS A BOGUS KNOCKOFF AND SHOULD BE AVOIDED! I hope this is informative and corrections and comments are always welcome. Ref 135.027 All 4 references appear to share the same case, 5.5mm crown, hands and matte bezel. The caseback Seahorse engraving appears to be the same on all 4 references as well. Casebacks on the 135.027 and 136.027 are flat. Seahorse is lightly engraved as shown. All original legitimate cases have notches at the 3 and the 9. This notch may be missing on bogus examples. 136.027 165.027 For me, the hardest of the 4 to source. Dial now includes "Automatic". Note the TT location on the dial is slightly different from the 135s Caseback is now domed to make room for the movement rotor. 166.027s THIS IS THE BOGUS 166.027!! It's plentiful, it's cheap. This was the first SM120 I bought. My mistake cost $500 before I realized there was a difference. Other than a Caliber 565 movement taken from another Omega, it's all bad Glossy bezel, wrong 4, different bezel font. Hands too short, Dial fonts and layout (Automatic is compressed). Black date on white background from the borrowed movement. Caseback seamonster is incorrect. Engraving is too deep. Crown too small. No case notch at 3 o'clock and 9 o'clock.