"In 1948 Chevrolet was the first of the volume manufactures to offer a 4-speed synchro-mesh, helical gear transmission as regular equipment. It was an extra cost option. As a result of this complete redesign, the 4-speed transmission case was wider, higher, and longer. It was still of the same rigid, box-like cast iron construction as before. So that the new 4-speed transmission could be mounted interchangeably with the 3-speed transmission it was necessary to increase the length of the 3-speed transmission case and main shaft." (from a 1984 article about Advance-Design trucks in the Light Commercial vehicle Association magazine)
