Back to this again over 1 year later...I found a serial number/date list for the H&R 999 Sportsman...link here...
Harrington & Richardson Serialization
Mine is a 6", with the vent rib barrel flat nose hammer, made in 1970.
VERY light SA and actually pretty good DA, and a good shooter. I have several 9 shot .22 revolvers - one of them a High Standard Sentinel steel frame .22 magnum snub - because I think they were a really good little gun to have around...and there are speedloaders for 9 shot .22's that seem to fit most of them...made by HKS. Got a few of those, too.