I asked the seller how we know its a 66 without extract and he replied with the following:
"We know it's a 1966 by the Serial Numbers on the movement which are in the 23,200,000 range. Omega released their serial numbers to the public at some point.
23,000,000 - 23,999,999 1966
Judging by the serial number I would say its early 1966"