Any idea what this is about?
I think it's about WW1 draft registration in 1917.
See this newspaper article written after registration day in South Dakota:
Here's the clip from the Freeman Courier june 14, 1917:
The line seems to have all women. I think it's for women's voting. The 19th amendment was passed in 1920 giving women the right to vote.
I think Gary is on the right track. Whatever's going on, it's a ladies event.