Many thanks Jack. Much appreciated! The server accepts attachments where the longest side is no larger than 1000px but it will try to render sizes a little larger than this. If it can't then you'll get an error message.
It's a shame that the postmark isn't very clear but no matter, what is clear is that 828 was definitely up and running in Dec 1943. Would it be possible to fiddle with the quality so that the Bann stamp is a little clearer? I assume that the "43" has now completely faded from the postmark?