Anthony Buckeridge did much the same with Jennings. He tried to make it up, to avoid dating it to a particular time. (It doesn't work for me, "Fossilised Fishhooks" sounds totally 1950's even if it shouldn't do. And The Thuggery always come across to me as early 1970's, which is odd since the book was published in 1965, but maybe it's because I was too young to take much notice in the 60's.)