Slightly old thread, but I have not seen it before now.
It´s a nice watch. I own a similar, also with "9" rather crudely stamped in front of the usual reference number 14704, and equipped with a cal. 552.
I bought it some years ago and first thought I had inadvertently bought a franken. As you mention, a 14704 is supposed to hold a cal. 591, not a 552. As far as I have been able to work out, the 14704 was the only reference ever being equipped with the cal. 591, of which only app. 60.000 were produced - all around 1960.
Apparently, the cal. 591 was developed as a less expensive automatic no-date caliber, intended for entry-level Seamasters. Being of same diameter, the 591 is slightly higher/thicker than the 552 (5.35 mm compared to 4.5 mm), leading to the domed caseback of the 14704.
Since then I have seen adverts for identical watches pop up now and then on the internet, albeit with some variation in dials and hands.
This has led me to believe that the ref. 914704 is not a franken. I think that Omega had a surplus of 14704 cases after they decided to terminate the production of the cal. 591. I think they equipped these with cal. 552´s instead, adding the "9" in front of the "14704" to distinguish them from the earlier production.
I have not been able to find any information regarding this from Omega. I haven´t seen enough of these 914704 to establish a period of production, but the ones I have seen so far all had serial numbers on the movement dating them to 1962. I have no idea how many were made, but I have a feeling it´s not many. Maybe one day I will spend the money and ask for an extract of the Omega archives.
Regards,
Woodward
