Sorry, I can't offer help as I'm one step behind you. Full grid but no idea where to put lines to make the geometric shapes. I presume the link( 12,5 ) is a P............ s .....but can't see that either.
Back to the dreaded grid- stare.
Oops ! I haven't drawn a 'c' as to my mind drawing a 's' and a 't' gives the 4 shapes referred to in the preamble. If I am right the thematic word is formed from the two letters adjacent to each of the edges of the 't'.
And so there should be (I realise now that counting the large 't' in addition to the 3 smaller ones inside it which are created by the 's' is an error because so doing makes FIVE shapes, not 4, and to get the correct number the 2 'c's in the Wiki representation are required) !! Another waste of a stamp !