Fake?

It's a bit hard to equate this caliber with some of the earlier models (500 series for example) where all of the points you mention were pretty well standard.
However, Omega didn't always engrave serial numbers on movements, and some examples simply had a printed plastic disc in an insert with the data on it.
I think this caliber may have been the result of the earlier quartz crisis and Omega was still trying to get back on their feet, thus the use of ETA calibers, which also explains the difference in the engraving compared to the older style used by Omega.