Hi Alan,
In the AMAC guff for their 1923-1924 range of carburettors they say there was "... a special carburettor made for Douglas "CW" machines. This is known as the model "T. 30. V.D." ..." Drawings and specs attached below.
"T" means 7/8" throttle valve, and presumably "V" means vertical, but beyond that I know nothing. I've seen the little 3-4 digit numbers on AMACs but I don't know what they mean.
Re measuring jet size. I have a set of extremely small number drills that are ideal for the job, but there is a cheaper and easier way. If you can find an appropriate sewing needle (a darning needle is good - I haven't told anyone where I find mine, but another member of the household looks after it for me), you can gently insert it in the jet, mark the spot where it comes to rest, and measure the diameter of the needle at this spot with a micrometer. Easy to compare jets because the needle will drop deeper into a larger jet.
Cheers
Leon