Slazenger stopped making the Slazenger Demon racket with the fishtail grip in about 1930, but continued to make Slazenger tennis rackets called the Demon for at least another 50 years. Also Slazenger had a ball named the Demon, again many different versions were made over the years. The one below is from 1935, and the packaging design used inside the box is downright scary in my opinion!
Interestingly at about the same time the popular fishtail Slazenger Demon racket finally went out of production, Dunlop started to produce the Dunlop Maxply, which quickly went on to be the most popular racket of its time, and be made again for about 50 years (1932-1983 approx). So 2 British design classics dominating the racket market in terms of popularity and sales in the UK for about 100 years amazingly.