If you have a 1950 1/2 ton truck, there was a mount at the rear of the 3-speed manual transmissions that attached to a frame cross member. I would suppose that the rear transmission mount served a purpose on pre-1954 trucks. However, Jim Carter does not show a transmission mount for a 4-speed (that weights about 3 times more than a 3-speed).

The Master Parts Catalog says the transmission mount is for 1/2 ton trucks through 1953.