I am not a dedicated FDC collector, but if encounter one with a stamp I like or is of topical interest I may get it. All the FDC's I have (about 45) have "FIRST DAY OF ISSUE" with 2 horizontal bars above and two below the text as the cancellation, except the Susan B. Anthony cover below. The envelope is postmarked Washington, DC August 26, 1936, which is the issue date for Scott 784.
My next oldest FDC is the 1 cent Franklin D. Roosevelt and Hyde Park (there are 4 stamps on the cover but not a corner block), Scott 930, issued July 26, 1945. It has the "FIRST DAY OF ISSUE" cancellation as described above.
Did they not use a "FIRST DAY OF ISSUE" cancellation until sometime after 1936 or could there be another intriguing explanation?