This is a hold to light postcard published by J. Koehler, hand dated 1915. Though not postally used, I had to buy this postcard. I came across it on eBay for sale buy it now, 4.99. The seller did not know they had a hold to light postcard, or else they would have been able to get a lot more for it. When you browse on eBay for any Chicago HTL postcards by Koehler, the least expensive one you can find is 49.99.
Page 7 of the McClintock newsletter put out by the South Jersey postcard club talks about the different series Koehler made. There are a total of 12 for Chicago. http://www.sjpostcard.com/ARCHIVES/2010PDF/052010/052010.pdf
The card is not trimmed, my scanner just cut off the left hand side of it. It is a very cute card showing all the swimmers in Union Park. To see other Chicago HTL cards, visit the Chicago Postcard Museum here- http://www.chicagopostcardmuseum.org/hold_to_light_LOBBY.html