Prism finder not fitting Rolleiflex 2.8f

Hello,
I recently bought a prism finder/viewer for my Rolleiflex cameras. It fits both the Rolleiflex 3.5F and the Rolleiflex Tele (tested with a friend), but not my 2.8F. The mount seems completely identical.

I know the finder came with an early 3.5F, and my 2.8F is about 20 years newer. Has anyone else ever had this issue? Is it possible to fix it with a small modification?

Thank you for your help!