After the first Citizen automatic in 1958, which used a conventional swinging weight rotor to drive it, the company launched its ‘Jet’ rotor, with a distinctive circular gear. By the mid-1960’s Citizen dropped the Jet and returned to the oscillating weight design – possibly because it proved more reliable, but most likely because it allowed thinner movements to be made.

One of the earliest models to use the oscillating weight is the ‘SM’ Autodater, which came in just two date only flavours – 17 and 21 jewels – and is built around the 2410 movement:

These examples typically illustrate the different styles of the day – the 17 jewel model has lumed hands and hour ‘dots’, whilst the 21 jewel model has the fancier bezel. And the backs also show two styles – the 17 jewel one from February 1966 is on the left, whilst the 21 jewel version with a more modern looking back is in fact a little older, produced in September 1965:

These were not high end watches, costing  JPY6,800 (17j) and JPY7,800 (21j) at the time, so the movements are plain, with no special finishing. Notably the rotor on this early auto is attached with a central screw allowing removal by a screw driver – this was later changed to a different design. Typical of Citizen’s automatics these hand wind as well – note the movement number stamped under the balance wheel in the second picture:

The later rotor screw design for comparison:

Date changing on the SM Autodater is novel – when the crown is pulled out to move the hands the date is changed by rotating backwards – anti-clockwise – which is something I’ve not seen in any other watch.

Finally, why call this the ‘SM’? Without the benefit of detailed translation, it looks that this may have been an abbreviation of ‘slim’ – reflecting that this design enabled thinner movements and cases.