I don't know the answer. The only thing I can think of would be the legal problems both the Zeiss Oberkochen and Zeiss Jena had for a while about the rights to use tradenames in certain regions. I have never heard about the "T" for coating being included in these legal matters, but it's a possibility. It it was quickly settled, that would be a reason why it isn't really known. Another explanation could be that the "T" was worn out in the engraving machine and a dot was used for a few lenses. Only guesses.