Possibly PT means the frame is pro stock (typically lighter static weight) while the TK is the generic version sold to the public which has a higher static weight as most would play it without customizing.
I gather TK codes are for a variety of things. Mold shape, handle shape (TK76, TK82, TK82S), and grommet codes (TK55A/1, TK203, TK293A ... possibly other things too?) so it could just stand for 'Tennisschläger kodex' or something. I read somewhere TGK, TGT codes for frames refer to where they were made, I think they only are used on frames.
So my understanding is my Radical Tour is a TK (where) 57(what mold shape) A-6 (what layup materials)
I welcome correction if anyone knows better.
@vsbabolat
It makes me wonder if the codes were issued consecutively. My radical tour is a TK57A-6 but the grommets are TK55A/1, so do the grommets and drill pattern date back to another frame before the TK57 mold?