So I have seen the Stewart speedo swivel with a 1801 number but this one has a 12003 number as well. It is the correct ratio of 1: 2 1/2 ... but what does the 12003 number mean?
Ask Russ Furstnow. The correct number for most Stewarts used on T's is 1913.
Mark, The numbers that are cast into the housings, both upper (1802) and lower (12003) are Stewart part numbers. Normally, the model of the swivel is "stamped" into the lower half of the swivel around the locking nut or just below the upper half of the swivel. The stamped number you are looking for is "1913" for a right hand drive swivel. If your swivel is stamped "1959", you have a left hand drive swivel which was used on Chevrolets.
I hope this helps,
Here's a Stewart 1913 swivel that I restored. You can see the stamping that Russ mentions in the photo :