I'd always understood it came from Frege's Begriffsschrift, a small descender from a horizontal line (part of a larger diagram), so like a short wide T, then lost its right arm later. Would you have some examples? (a quick google images shows nothing like that ...)
It was definitely a thing in Soviet works on mathematical/symbolic logic. But here's an example from Curry [0]. As I said, it was mostly a stylistic choice.
