Maybe a good time to ask, but I'm wondering if my 4-speed transmission in my '46 is a SM420?
John - Your pictured 4 speed w/main case casting number 590474 is the SM420 predecessor - GKT-134, otherwise affectionately known as a "crashbox".
It's a non-synchronized 4 speed with straight cut gears where the SM420 2nd through 4th gears are synchronized and have helically cut gears (1st and reverse are un-syncrhonized straight spur gears).
