Well, when I first removed the back cell I found this ring, then the flint, and then the crown.
(from inside / outside the lens).
Then I thought, another petzval badly assembled ...
Then dismantle the front cell and found the same ring and the front achromat.
These two rings act as "adjustment" of the elements of each cell. Assemble just before screwing the last ring that closes the housings and makes the flint/crown and achromat do not move and have no slack in their respective housings
Searching and searching, I found this thread:
https://www.largeformatphotography.info/forum/showthread.php?70668-Bausch-amp-Lomb-Projection-Lenses
in which
rob_gervais, exposes the following in post # 5:
(I canīt see the images, it would help to know if the lens is like mine in 22in)
The order of the elements is the same as the one I found. - ring, lens, lens -.
Rub mark that he found, I think it was caused because the flint and the crown were incorrectly placed and rubbed against each other. (If you place them with the curves in the correct way, but without a spacer ring, they only touch very little on their outer edges.)
Maybe never come out again, is more, if you put this ring between the lenses and screwed the last ring that closes the housing, sure at least that lenses will suffer chips on the outer edges.
I thought maybe this B & L did not have a spacer ring, but Garret's post has me puzzled.
Many Thanks, Rudi. The mystery continues ...