According to my database, N 629100, was a prototype gun and shipped Nov 1978. I have serial numbers starting at N611xxx thru N628xxx all shipped from Jan 1979 through Feb 1979! Here is a quote from Dr Jinks on the 629s;

....... the first production Model 629 had serial numbers that began N 629xxx these were basically test guns and guns for the writers. Although we shipped some of them to distributors such as Bill Orr’s company GT Dist. The revolver bearing serial number N748614 was in the second lot of 629’s that were manufactured in March of 1980. This lot was for 350 units and were serial numbered from N748564-N748913. These were the first group produced for sales
jcelect