1985? Huh. Well for sure if there were 421a tubes being made in 1985, it wasn't Tung Sol making them, as Tung Sol was long gone by then. Cetron, who bought Tung Sol, continued to make some of Tung Sol's tubes like the 6336, sold under the Cetron name, for some time, maybe even as late as the 1980's. But not the 5998, I don't believe. So perhaps WE had to start making the 421a, and thus the slight design change.