Hi Mike,
The dates on the postings are taken from either, the rear of the postcard if there is any printed info, or taken from the postmark as you have quite rightly guessed. Of course the photo's could have been taken prior to printing and posting hence why I say 'thought to date from'. I'm trying to be as accurate as possible.
It's just nice to put a little back into the forum, and hopefully spark a few memories for people.
Ian.