The tachos for the AT2/3 and CT2/3 look identical, but are listed as having different part numbers (316-83540 vs 315-83540). There are boxed, new of each on fleabay, so maybe just not a typo.

Does anyone know what the differences are?

Many thanks.

12v bulbs vs. 6v bulbs

(All 1970's Yamaha gauges I've bought new came with the bulbs and harness)
 

Thanks Mark. 

Well there ya go. Saves me trying to find the answer to an impossible question.

For those that don't know, the Yamaha gauges are very interchangeable between years and models as they are calibrated the same inside the gauge.

For example, a 1969 125 tach and speedo will work on a 1973 360 just fine...  the redline markings and other details won't be correct but the gauges themselves have the same internal calibration.    2000 rpm will be 2000 rpm and 35 mph will be 35 mph even though the engines and wheel sizes are different. 

Yamaha calibration is done on the drive side.  So if a bike has an 18" front wheel, the speedo drive gears in the wheel are calibrated to spin the cable at say 1000 rpm at 60 mph (I don't remember the exact calibration "cable rpm per mph", some gauges have it printed in tiny letters on the face).  A bike with a 21" front wheel has speedo drive gears in the wheel calibrated to spin the cable at that same 1000 rpm at 60 mph (or whatever the numbers are).