Sometime this summer, I found this plough plane blade at a yard sale. As you can see, it's marked "A. Hildick Sheffield" with "Diamic" in a diamond.
Founded in 1846, Aaron Hildick operated out of the Woodside Works on Rutland Road in Sheffield. It's "Diamic" trademark goes back to at least 1900.
In 1948, Hildick acquired Henry Taylor tools. By 1951, the company also listed Charles Taylor's (Sheffied Tools) and John Jowett on their company masthead:
The new, larger company continued to use the Henry Taylor name on some of its products until 1974, when it officially became Henry Taylor (proprietor Aaron Hildick), and five years later Henry Taylor (Tools) Ltd., incorporating Alan Hildick. It's thought this name reversal may have occurred because the Henry Taylor name enjoyed better brand recognition. Henry Taylor Tools is still around today, and still uses the Diamic trademark. I believe that they're still made in Sheffield, although I note from some wood turning forums that there are complaints about decline in quality of the tools.