I fished out three lovelies from a bargain bin last weekend. These appear to all be from U.S. flagged ships, cancelled in the summers of 1934 and '35. They are on letterhead of the SS President Harding, President Monroe and President Polk. It looks like whomever created these had the ships' postmasters provide not only the standard Sea Post cancels, but all the auxiliary markings that were in use. Some interesting stuff here! Several of these saw very infrequent use and would probably be quite rare on cover. I don't really understand the Harry Deitch marking on the SS President Polk sheet.
Was it common practice to cancel courtesy sheet like this? Can anyone post other examples?