If its a 4 speed, it should be a SM420 which was an extremely tough transmission used from 48 to 67. I'm not able to identify it from the numbers. Try pulling the stick all the way toward the driver side door. If it goes into reverse when you pull it toward the seat, its a SM420. If it can be moved toward the dash as well as toward the seat, it could be a 5 speed Clark or New Process. 5 speeds were rare in buses unless it was a military bus.
