From the summer of 1983: the all but ubiquitous RETURN OF THE JEDI PROMOTIONAL POSTCARD.
These seemed to be handed out everywhere... especially when Darth Vader was making a promotional appearance to promote the Palitoy action figure line. Although the card itself promotes the Odeon cinema chain and not the toys.
The Dark Lord, clearly with time on his gloved hands after the defeat of the Empire, kept himself busy by touring the nation's toy vendors. I seemed to run into him several times that summer: once at a toy store in Colchester (long gone over twenty years ago) and once at a branch of Debenhams although I know not where. I seemed to recall he was accompanied by a menagerie of other aliens for the second encounter although I don't remember what and there is a good chance they were not even WARS related but just seemed like the sort of aliens that might lurk in Jabba's palace.
The Darth stand-in would, as I recall, even autograph these cards and I'm pretty sure I have a signed one somewhere. Needless to say, I doubt very much that it was Dave Prowse beneath the mask.