This might possibly be the 7.6×66 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