Just wondering what if any the improvements are between Reverb model years and how easy is it to tell the difference?

Not Stealth version by the way

I think the earlier ones had teething probems but the newer ones with black collar seem better. Ive had no issues with either of mine in 2 years

Collar colour makes no difference as the later versions are also available with silver collar. Look at where the hose emerges from under the saddle, the earlier versions have a plastic boot on the end of the hose, the later ones are metal.

to my knowledge there is the A1 and the A2. the A2 has a black speed adjuster on the remote and the A1 silver. they push a different amount of fluid so are not compatible.