Later convention for Transformers was to use Tx or TX... As in LOPTx (line output transformer). It was to distinguish from any other inductance which used the L as a marker prefix. (That said some early German diagrams had all inductance units marked with a W the link being the unit of measure the Weber)
I have confusingly also seen a coil with a dust core given an I prefix on a French CCt diagram
And some Toshiba transistors in a radio circuit prefixed with an S, what's going on there?