I don't think they are that uncommon. I have a 4 screw 10 no dash dated to Oct 59. # C45939X. According to the book, the 10-1 was the introduction of the Heavy barrel in 1959. So the standard barrel was still a "no dash". The 10-2 was the introduction of the left hand thread on the extractor rod, in 1961. So you could find a no dash from 1957 (start of Mod # marking) until 1961 with standard barrels. I imagine quite a number could have been mfg in that time period. Check page 174 of the Standard Catalog of S&W for all the engineering & production changes..