I recently purchased a post card of Hatfield House which was photographed in 1886. And how do I know? The card was published by Valentine & Sons, Dundee, and was numbered 6470 - the number being the negative number. St Andrews University have a major post card archive, and have produced a list relating the post card number and the date the negative was taken.
I am building a list of numbered Hertfordshire post cards and if you know of other numbered Valentine cards I would love to add details to my list.
_____________________________________I have now (February 2013) set up an illustrated list of the Valentine cards I have of Hemel Hempstead, with higher resolution images of the cards and also information on their backs as part of some research to get a better understanding of what the publisher was doing at what dates.