I have this butt plate for spare. It´s similar like Mannlicher Schönauer pre-WWII but lower tail end is about 10 mm (0.393") shorter than normal one. Screw spacing is same as original one. I´m sure it´s very easy to tig weld a piece of steel to make tail to it´s correct lenght for M-S.

Asking 120€ + postage.