John Bull was the brand name of a fairly primitive children's printing set based on the letterpress printing principle. The set comprised long strips of black rubber letters and numbers (more or less, depending on the size of the box), a small red plastic rack for composing, and an ink pad for printing on the paper. There was also a pair of tweezers for handling the fiddly letters, which could get quite inky with use.
The rubber strips were cut into single letters and numbers with scissors, and composed into words and sentences on the rack. The letters were in reverse so they could print the right way round when pressed into the inky pad provided and stamped onto paper. These sets, which had been around since the late nineteenth century, were still popular Christmas presents for children in the late 1950s.
The distinctive figure of John Bull was a variation of an original creation by Dr John Arbuthnot in 1712, popularised by British print makers and later by writers and illustrators at home and abroad. He is sometimes used to refer to the whole of the United Kingdom, but is viewed at least by Scots and the Welsh as specifically English rather than British.