Cards used for things other than originally intended isn't too uncommon... Old Put- E98's, many W575-1 cards w/different ad backs, W573 and many others. Some were "re-purposed" and others, it seems, are made generically allowing different stamps and ads to be added to the backs. Here is one from the D382 series that was re-purposed with a sticker on the back and a few others with stamps/lettering on blank backs....
