Seems to be at least 3 sizes. Dingo says he has a RBII and the dome is 4 1/4 including the gasket. I have a 3 3/4 (including gasket) spare dome and my corporal has a dome that measures 3 1/4 (including gasket).
What does the 3 3/4 fit?
Taller is appx, 4" with gasket
Shorter is appx. 3" with gasket
There may be slight manufacturing anomalies and vintages that gave slightly different heights, but they fit either RB11/Deputy, or Corporal. There were no others, unless you get into the 66, or Seniors.
3 3/4 would be for RB11/Deputy. I just went up and measured all my Rota-Beams. They fluctuate installed by close to 1/4" (+ or - appx, 1/8"). This could be manufacturing tolerance in dome height, gasket depth, or installation tightness of dome when twisted down on dome retaining clips.
I also have 2 original version Seniors in which there is a difference in dome height.
I guess we have to remember that especially in the early Rota-Beam days, there were no CNC machines making these products.