The engine is most likely a 3.0 M103 (Not OM, that means "oil motor" aka diesel)....the 2.6 would be a bit weak to pull around a W126....and about 75% of the M103's in the USA are 3.0 liter.

I am not actually sure where the ID number is on the M103, it might be along the top of the block either on the side, or front.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)