Mystery Mail from World War I Soldier

The above postcard or carte postale is the oldest item in my grandmother's postcard collection.  A W. W. Allen sent the card via soldier mail from Paris, France, to Mrs. Frank Bryant of Los Angeles, California.  The front depicts an early 20th century street scene at the Notre Dame Cathedral in Paris.  The message says, "Paris 4/13 [April 13th] - I'm having a short vacation and a wonderful time.  W. Allen." 

Mrs. Bryant (nee Mary Ellen Ingersoll, 1862-1936), originally from Bloomington, Illinois, relocated to Los Angeles with her family in the early 1910s.  She was my grandmother's grandmother.  Mr. Allen is not a known relative.  So who could he be?  Perhaps he was a neighbor or a family friend.  There could be another connection.  There is a later postcard postmarked November 25, 1930, from Alice Allen of South Bend, Indiana, to Mrs. Bryant which starts with the salutation, "Dear Cousins." 

A little sleuthing revealed a Mrs. Alice Allen living in South Bend in the 1930s, but I could see no family connection to the Ingersolls or the Bryants.  Unfortunately, I was unable to shed light on the identity of Mr. Allen.