I second google. I believe WT came onto the scene quite a bit later on as far as insulator manufacturing goes. That is a CD 154, sort of the "final standard" as far as insulator designs go. WT made lots of different styles.
Whitall-Tattum should have been making insulators before 1922, for some come in SCA glass, which was phased out by 1920 due to the supply of Manganese being cut off by the Germans in WW1. Whitall-Tattum Co, was around since at least the 1880s, though until about 1901 they were Whitall, Tattum & Company-- notice the comma, and, most importantly, the ampersand. Otherwise, the buy-out date sounds correct, for I also read somewhere that Armstrong's succeeded them.
WT has been around since the late 1800s. They expanded into insulators in the 20's and were bought out by Armstrong's cork in 38'. However, even after Armstrong's bought them out, they continued to put the WT name on the insulators until sometime in the early 40's. I have those clear CD 155 WT insulators with 1941 and 42' dates on them, but on the rear skirt they have the Ⓐ in a circle indicating Armstrong. Not too long afterwards they replaced the WT name with Armstrongs.