What letters?
The ones from post 77 refers to the continents
S=SOUTH N=NORTH ... C=CENTER P=POLE I=ISLAND

The other letters from post 70 have no signification. 1 to 80 and A-Z and then add something after. Using just numbers is confusing.
this is considering that we keep the grid with the 500x500 km for each area.



I worked on the elevation:

Attachment 66615

Some areas are without colors, they are yours if you want.