The serial number that I have always heard as a cutoff for non-tempered cylinder has been 316648. Otherwise, Saxon Pig summed it pretty well. In addition, QC went to **** some time in the mid 60's, certainly by 1967 or 1968. Probably others on the forum could tell you the date a little better. If you have a choice, you should probably set a limit at the early 60's, although I would probably specifically go for a 10-3 (1961 HB) or later to get the LH extractor rod and .125" front sight, or, in std bbl, 10-5 for the same.
I'm not sure, but I think that QC became reasonable again at some point. Others can advise. But 10-3 thru 10-6 in the early 60's should certainly be OK.
Now that I think about it, I bought a 19 in the mid-60's, and it was fine, but I think that they may have been watched a little more closely. It's probably best to do your own QC.
Good luck!
BTW, you are not particularly likely to get a nice old 4" 19 RB, unless you put a 4" bbl on a frame originally sold with a 2.5" bbl.