Oh, I want to know how to modify the game. Please explain to me if it's okay with you.
First off -- be careful and back up any files you plan to change before you modify the game, as it is possible to leave the game in a state where it won't run correctly. If all else fails, you can always re-install though.
The data file you want to modify in this case is named "AlsTeams.txt", and is found in the folder DATA\BASE\ under your install directory.
There is an Excel spreadsheet, known as the 'data workbook', for Wacht am Rhein, that will help you understand the format of the data files. You can get the workbook here: ftp://ftp.matrixgames.com/pub/CloseCombatModernTactics/tools/WaR_Data_Workbook.zip
In this case, you go down the lines of data in AlsTeams.txt file to find the line for the "Command M4A3". You then want to change the value in the 11th column (column K if you're looking at the workbook). This value controls which graphic is shown on the battle group screen for each team. The current value is 25, but if you change it to 36 (the same value as the normal M4A3(75) entry), you should see the expected graphic on the battle group screen.