Where do the white/black letters T K T P stand for?

They line up with the period names.

So from the bottom:

• P - Permian
• T - Triassic
• K - Kretaceous
• T - Tpaleocene

(no, I have no idea either)

K is indeed for Cretaceous (Kreide in German)
T stands for Tertiary, an old term for the Paleogene