Following the advent of the Airplane Mail there have recently been rumours of a Submarine Mail Postage Stamp. These reports, no doubt, had their origin in the Submarine Mail service established some months ago in the Canal zone. For the purposes of the Fourth Liberty Loan, the United States Government pressed aeroplanes into their service. Improving up that idea, for the last loan a submarine, C3, was used, and with the best of results from a subscription point of view. The service was known as one of the "Ocean to Ocean" variety, Christobal and Balboa being, respectively, the receiving and distributing points. As a matter of fact, no postage-stamp was issued expressly for the occasion, but a special postmark was applied to the stamps on the envelopes, and these so-called "Submarine Stamps" are being energetically sought after by collectors.I wonder how many were produced, and whether they are still being collected.
Reference: Boy's Own Annual , vol. 42, p 83