Re: Model 48 Topper
'H' = 1947, as you suspected.
As to the 16 ga - from my experience in researching and watching H&R pricing, over the past decade and more, that bore is quite a bit less desireable due, primarily, to the paucity of supply. My opinion, based up the above qualifications, is that 16 ga is a liability with regard to price/value when selling common, lower price range shotguns. Some highly collectible pieces may benefit from less common chamberings, but not the H&R Mod. 48, which morphed into the 58, 158, etc. and later models.
I'm inclined to agree on the 75 to 80 bucks you are thinking of.
Jim Hauff ~ H&R Collector In Memory of Bill Goforth and Jim Ritchie