Problem mounting PA-Curtagon- R on Leicaflex (standard)

I have a Leitz (Schneider-Kreutznach) PA-Curtagon-R shift lens.
The box says Leicaflex, but no matter how hard I try, I can't mount it on my Leicaflex (standard)?!
I align the red dots and it fits, but I can only turn the lens ever so slightly (and then it is almost stuck).

Do any of you have a similar experiences or maybe even a solution?

Cheers,
Kasper