NOS exhaust pipe ID

[sobmstr]

Looking for help identifying some NOS exhaust pipes I have in my collection.  They came from a BSA/Norton/Ducati dealer that was a family friend.  I've had these & other NOS parts for 40 yrs & have begun selling off.
The pipes shown appear to be a C15 250cc style, but the rear mount is different.  The measurement from hole-to-hole front to rear mount is 22 1/4".
I do not have a bike to compare to so perhaps someone can help identify.

Thanks