When I first got my SM300 ....I took it to be a 1964 as I recall my Father got the watch roughly in that year. So 1964 it was, until I opened the back to read the serial # which according to the Omega serial # chart of 23 mil.....put the watch at the 1966 year. I have recently found some interesting information that seems to make some sense from another watch forum that may be of some interest to some other SM300 owners. Here is the following (quote) from that forum>> (((((but to clarify the serial number, it appears from my experience that sm300's dont follow the same rules as most omegas in the same way that speedmasters dont. I have owned 3 SM300's from 1964 and still own 2, a 165.014 and a 165.024 both stamped -64 after the reference. All 3 of these have had 23 million serial numbers in them as has every -64 SM300 I've ever seen. On that basis I would be confident in saying that yours is a 1964 model. A very nice family heirloom.))))) end quote. --------------Hope this may serve to clarify.......if indeed this information is accurate....cheers!