The Lost Battalion
On the right is a Harley-Davidson and on the left an Indian. The Indian looks to be a side valve model so is probably a Scout from the 1920s. The Harley could be closer to the correct period. I will check.
The Harley has electric lights so is a Model J - like this one http://motorbike-search-engine.co.uk/classic_bikes/harley_davidson_11j.jpg but could be anywhere from 1915 to about 1924.