I am hardly a 1917 expert, but I have two (an original and a Brazilian) and have some opinions.
I think the earliest guns with the dished stocks, grooved hammers, and GS inspection stamp are preferred. They are all numbered under about 40,000 if I recall correctly. They date from late 1917 through spring of 1918. After that, in order to increase production, niceties like the scallops on the stocks and the grooves on the hammer were jettisoned. At some point -- not remembering exactly when as I type this -- the inspector's initials were replaced by a flaming bomb stamp.
I have 13546, which shipped in January 1918 -- probably in the first week, based on production stats and other ship dates. I would have to letter it to get the exact day.