Drew is pretty much on target. I would just add that "rare" is perhaps the wrong word to use for an early production 1917 Army. About 150,000 of these guns were produced in all, and then 60,000 more that went into the commercial market or were shipped to Brazil. In all its varieties the 1917 outnumbers all other N-frame revolvers produced before 1950.
But there are classifications within the 1917s that have special appeal. The earliest 1917s had the radially grooved hammer, the dished service stocks, and the GHS cartouche on the left side of the frame in front of the hammer. These are usually felt to be a little more collectable and a little more valuable by collectors, and you have one of those. But 1917s had at least one of these features up to about serial number 25000, and all of them up to about 15000 if I recall correctly. That means these features are less commonly seen in surviving 1917s, but should probably be called "scarce" in the market rather than flat-out rare. I usually think "rare" should be applied to models or subvarieties of which fewer than a thousand units were produced, perhaps 1500 tops. There may be some models that could be considered rare because they are widely collected and little seen, even though as many as 4000-5000 were produced.
Nice gun.