It's a "T" with a line in the middle and a line at the bottom. The legend I heard was that Zeiss Oberkochen wouldn't let Jena use the "T" and they modified existing lenses this way so they could be sold. As I said, a legend; I've no idea if it's true but there you go.
That'd make sense, I've heard about similar stories just never seen a marking like this one before.
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