The ref is ST 166.0177, available as international collection b/w 1974-1978. Movement is caliber 1012, 23 jewels.
No connection to SHOM to my knowing.
The on featured in your pic is having its crown in the wrong position, it should at the right hand side of the watch at 3 o'clock position instead of 9 .
IMHO, the 70's Omega diving watches are more appealing in the way that their appearances are very retro. Most popular one is of course the Ploprof with water resistance up to 600m. This is due to its unique case design. The rest of the line are 120m, 200m and 1000m which are less featured ( dont know why)
I think you got it wrong with the seller claiming this watch is Omega NOS. I think it could be a case of the watch being send back to Omega for an overhaul and they decided to change the hands with NOS. Perhaps you may want to verify this?
I'm offering my 2cts worth here. If you are really into watch collection, then it doesnt matter what price you paid as long as it's within your budget. It all depends on the timing.
However, if you are buying it just hoping that it can change hands with a higher value than what you had paid, I strongly discourage the idea as it can be quite painful.
Good luck with whatever your decision...