Early grips were numbered in pencil. Sometime before WWII they started stamping the numbers, but I don't know exactly when. My 1917 vintage .455 MkII has them in pencil and on my 1942 Victory model they are stamped.
I should also mention that the reason they put the serial number on the grips was because they were closely fitted to the gun before it went to final finishing. Since target grips didn't have to be so closely fitted, they weren't numbered.