Gordon:
The true value of any gun is what someone will pay for it. In the case of a S&W 1917 revolver the asking/selling price often varies widely. "knzn" suggested $300.00. Others have speculated up to $1,200.00. The true answer is, it depends on who is selling the gun to whom, and where the transaction is occurring.
Recently I noticed a S&W Military & Police revolver being offered for sale on this forum. It was in the "C" serial number range, made around 1948 or as late as 1952. It was a nice, but not exceptional piece in about 95 to 98 percent condition (I'm judging from a photograph). I would have valued the gun at between $250.00 to $300.00, but the seller wanted (as I remember) $450.00. Now I can, and have recently purchased similar revolvers in the price range I stated, or less. But he quickly sold the gun in question for an unstated amount, but I presume he got his asking price.
So one could say that all Military & Police .38 revolvers made during this time period, and in the same condition are now worth $450.00 or more, not withstanding the fact that a careful shopper could buy such guns for half that, or slightly more.
As I said before, it all depends on the situation, but I'll stand on my $600.00 estimate as something one could expect to get selling the gun anywhere, and without special circumstances. If someone can get more that's fine.