Hey Rich,
You can find the location for the two letter code
Here. It's a 54 chart and the same for 1953. The only difference is the first letter, year "M" changes to an "L" in 1953. The second letter(s) G & H remain the same.
There is a casting number 3969535 for 3/4 & 1 Ton from 47-57 w/ 4.57 ratio, teeth count 32 & 7.
