It is the nosepiece that comes with the new ASI6200 (https://astronomy-imaging-camera.com/pr ... -pro-color). This camera is supposed to be released sometime in January. It was also used on the ASI094 and ASI128, that have been discontinued.
I did find one vendor that carries this nosepiece (https://www.365astronomy.com/zwo-m54-ma ... ength.html).
To NPB, why the concern? Are you trying to use a focal reducer or a flattener and have a specific spacing you are trying for?
I am using this with a ASA corrector and need a precise backfocus. Even though the ASI6200 cameras won’t arrive until January, I would like to get a PreciseParts adapter (corrector to Clicklock) on order.
I think it is 2mm as well, but could not find a specification anywhere. I actually just ordered this nosepiece from 365astronomy to get an accurate measurement (and have a spare nosepiece).