Hi Kevin:
To the best of my knowledge, T204815 would not get an "S" as it was not a self propelled gun. If it was converted from a gun to a GPO vehicle then it was re-lettered. If it was built at the factory as a GPO vehicle then it was a "T" from the get go.
I hope you will post all your research when you're done... its invaluable for historians, modellers and restorers alike.
Mike
