IFAIK, this originally referred to the small head diameter valve used in 1953-66 Buick V8 engines.
They may be referring to the head shape, or rather the stem-to-head radius, in which the head fits the stem like a "penny on a stick" or letter "T". This is the opposite of what hemi engines commonly use, the "tulip" valve with a big convex bell instead of a radius.