So far I've seen three sizes of rear cap for the Nikon RF lenses, not including the ultra-deep 2.1cm one, so that makes four.
The 8.5cm, 10.5cm and 13.5cm all use the same depth cap, the 2.8cm and the slower 3.5cm lenses use a slightly deeper size. The 3.5cm f/1.8 uses a significantly deeper one still and finally the 2.1cm uses that monster. I'm willing to bet the 2.5cm uses the same one as the 3.5/1.8.
Interestingly, the NKK metal rear cap from the chrome 3.5cm f/2.5 doesn't fit the 2.8cm f/3.5 but the plastic cap is the same between both lenses.
EDIT: I forgot to mention: DROOL! An amazing find!
Phil Forrest