Our American friends have done forensic studies on early Indians (and Excelsiors for that matter), and can plot out year-by-year changes pretty well, even noting the usual not-true-to-life-artistic-retouching that goes on in the various catalogue illustrations. And because Indian were producing bikes in quite large numbers there was little bike-to-bike variation in a given year model.
The biggest deviation from standard I can see in the bike in the photo is that the left hand petrol filler cap has been screwed into the right side tank!!! Shame on the owner. There were minor changes between the 1918 and 1919 models, the most obvious being that the broad black band between the gold pinstripes was replaced by simple gold pinstripes in 1919. When I was young, c1980, I bought an almost complete 1917 Powerplus in original paint (from the Age newspaper in Melbourne, at the crack of dawn on a Saturday) and the broad black between the gold stripes was still visible. The black is rarely seen on restored 1917 and 1918 bikes, and the photo is not clear enough to see whether or not it is on the bike here.
In 1918, the Powerplus was available with no lights (model N-18) or with full electric set (model NE-18), so this is the "bare" version fitted with acetylene light and a bulb horn.
Is the "5" on the front some kind of entry number in an event? Presumably an early 1920s photo.
Leon