Morning Ken and Welcome to the friendliest forum on the 'net!
With regard to the various Engineering Changes as delineated by the "-" markings, I would say that the Pre-19s (no model markings) are about the most valuable and the "-1"s are the rarest.
Generally, the earlier - number, the better.
-3's & -6's are pretty common by virtue of their duration in production and I think that the later guns make pretty fair shooters. Some -3's will be found with Diamond Stocks and these are considered desirable.
As far as saleability, I find the 2 1/2" barrel length guns command the best prices followed by the 4" and then the 6". Authentic 3" and 5" guns are rare and bring a huge premium.