Mounting a Rodenstock Grandagon 90mm f:4.5

Hi !
I just got this wonderful early 80's Grandagon 90 mm. What a beast !
I got it on a Technika lens board. I need to mount it on an MPP one.
The lens is on a Compur size 1 (black one).
I thought it was an easy task...
The rear group is fitted strangely for me : on the Compour is screwed a piece onto which the rear lens group screws EXTERNALLY. The piece has a ridge forcing you to unscrew it to remove the retaining ring.
On the Linhof lens board there is a flange screwed (like a device made to convert a size 3 hole into a size 1 one). This flange is not the same metal /finish as the lens board and is locked in place with something like Loctite product. It is threaded but I can't unscrew it...
When I mount the Compur on the MPP board the extra piece fitting into the rear of the Compur is unable to screw fully home, pressing on the retaining ring so I can't retain the proper spacing of the lenses.
So it may seem I really need the extra piece from the Linhof board ?
Anyone has experience with this fine glass ? Any clues ?
TIA !

I have an early 80's Grandagon-N but the f6.8 not f4.5. It fits easily. You should check the Rodenstock website

Is it the normal retaining ring glued to the Linhof board ? They are a little bit more substantial on Copal 1 shutters compared to O's.

These lenses haven't changed since the early 80's.

Ian