These next two photos provide some exciting clues:
We have a location for all of the most recent photos! Shippensburg, Penn. Hooray! This all but confirms to me that the Red Bridge Park photo also was in Pennsylvania.
Next is an even more exciting clue.
Names! First and last! I found Maud Geedy and Hazel Walters at age 16 in the 1920 U.S. Census in Shippensburg (their descendants are future candidates for having a photo or two returned to them!). The Geedy family was on South Washington Street and the Walterses on North Earl Street. These streets run about parallel, after looking them up on Google Maps, and are separated by about five blocks in between.
After inspecting this photo closer, note all the American flags that are attached to the arbor in the background. Might this have been Labor Day weekend in 1915? A photo in a previous post was dated September 1915. Labor Day was first celebrated in 1912. WWI had commenced, but the U.S. isn’t involved yet… (Actually, in reviewing some of the photos I’ve already posted, I don’t see those flags in the gazebo/arbor. This photo may have been taken on a different day entirely.)