7.5 X 66 MAUSER XPL (7.6x66 Maximum)
This might possibly be the 7.6x66 Maximum (W98) as described in Dixon.
DIMENSIONS:
BULLET DIAMETER: 7.70mm
CASE LENGTH: 65.90mm
NECK LENGTH: 9.20mm
SHOULDER LENGTH: 53.40mm
BASE DIAMETER: 13.90mm
RIM DIAMETER: 13.90mm – THE RIM IS NOT REBATED
RIM THICKNESS: 1.20mm