Public appeals like this one were screened during cinema intermissions by placing the slide into a special holder in the projector for a couple of seconds (any more and the slide would crack!).
Many of these 'appeal' slides (and ones advertising upcoming film screenings) had a blank space at the bottom for the projectionist to add the latest details in removeable ink. Some blank slides were kept on hand for 'on the spot' messages like an appeal for a doctor if there was a medical emergency in the cinema.
Compared to many of MAVtech's advertising slides this one is quite basic- but it would have gotten it's message across well. It probably dates to the 1950s/1960s- long before we had a national blood donation service, hence the lack of logos.
In the time before the internet 'watching the papers' was a great way to get the word out....
