OMG, how I used them. First was in 1972 when I met a WG in Green St Forest Gate. Just 10 minutes from where I lived!
Then there was a contact club for us married. Run by two women living in Golders Green. All done by snail mail. They sent out (for a fee) lists of married ladies to the men, and married men to the ladies. Each on the list had a nickname. Like 'beauty', 'lovely lady', dark haired maiden' and so forth. Had to write to whoever you fancied, put it in a sealed envelope, and send to the ladies that ran the club who would forward the letters. In those days, accommodation address's the only way of getting your mail from this club. Can't remember its name? Only that it advertised in publications like Private Eye.