Just my opinion - but one tell to me is the condition of the bore. If it's a rough sewer pipe, might well have seen WW1 service, since almost all of them are. If it looks in pretty good shape, then probably a later rebuild. Not a sure thing either way of course, but seems to often be that way.
Also, how's the reciever fit in the stock? I've seen in a 1917 if shot a lot, it compresses the lug's wood in front of the reciever and tends to start riding further and further back in the stock. And eventually the reat tang of the reciever that you can see on top, starts engaging and compressing the back of the stock there - ultimately threating a stock-split there over time. If the gun and reciever haven't been shot much, there might be a nice big gap between the tang and wood, which would imply a low-round count 1917 (to me; someone may correct and say I'm full of **** on that; though hopefully nicely if so :) )
I like 1917's, I think they are pretty neat underappreciated guns.