I just purchased a 6" 66-1 like I have been looking for for two years. The S/N is 47K72xx. I come up with a manufacturing date of May 2, 1979 per the dates in the Std Cat for S&W guns. Does this date appear correct to y'all more knowledgeble folks? It does have the grips on it that have the left side cut out for speed loaders. I thought the cut out grips came out in 1980 so their date doesn't match the interpolated date.