T for Tens as in tension of the heavy kind?
I recall once having a tape recorder made in the Czech republic. Apart from the engineering being crap, the din sockets on the rear panel had some very odd signs against them embossed on the plasstic. From memory the screws had a circle with an upside down t and an exclamation mark beside them. the din for microphone seemed to me to be a tiny face with a zig zag line and a mouth like an O. I never did understand them. I'm sure if you were Czech it was obvious. It was made by Tesla, no not the inventor the excuse for a company bearing his name that exported to the world and most of the devices fell apart in a year. Brian