nose to the grindstone

As I was passing by this very fragrant privet I noticed a bee busily nosing around. Once I got the photo up on the computer I noticed how tattered its wings are, which made me think it was a very hard worker, i.e. had its nose to the grindstone.
Then I got curious about the origin of that saying. Here is what I found:
"In Sheffield, England, many of the preserved Georgian tool sharpening workshops have benches where grinders would sit hunched over grinding stones. They had to pay close attention to their work as a moment too long on the wheel could cause the steel to overheat and be ruined. They literally had their noses to the grindstone."