This would be a perfect solution, Pentax!
Unfortunately I haven't got a link for you which is a pity for me as well. I don't intend to be part of the exchange but at the price of postcard paper I would want to try using ordinary 4x6 paper whenever I want to send any postcard prints out to friends etc.
Try a search here on APUG. It has certainly been mentioned in threads/posts. I have a feeling that Dave Miller's posts( he used to post here) contain a reference to the pre-printed backing paper. However Dave is U.K. based so it may be that his source is/was also in the U.K. although I have to say that most innovative analogue stuff these days seems to be U.S. based rather than U.K. However companies in the U.K. are best at being "innovative" in one respect, namely with hiking prices.
It is of course possible to buy plain A4 sheets with sticky backs and use a computer printer to print the postcard back. You then simply cut the sheet to size with a print trimmer and stick it on.
Anyone else who has been a round for maybe 4-5 years recall the source of this backing paper?