PR (PROOF) vs MS (MINT STATE) & also SP (SPECIMEN) is only a designation of the TYPE of strike.
The NUMBER following is the NUMERICAL designation of the GRADE assigned to the coin.
Each portion (MS/SP/PR) & (01-70) speaks to a different aspect of the coin itself.
The first portion, (MS/SP/PR), tells the viewer what TYPE of controls or constraints under which the coin was struck. That is to say, what TYPE of strike it is.
The second portion, the number (01-70), is merely a numerical designation of an assignment or assessment of the coins condition, in other words, how much wear it has experienced. This numerical designation has a specific correlation to specific word grades.
Hence, taken together, you can know the TYPE of strike AND the CONDITION of the coin from those 4 characters.
I am using the Sheldon 01-70 system as my example. For CGS/LCGS just substitute 01-100 for 01-70.
I hope I didn't confuse the issue even more..... Just 2 independent aspects of the entire designation.