That's a 4Rxxxx model D6 with the D4600 engine.
It would be a s/n 4R2266-up after they changed to the large rear hub.
Be a 1945-47 model.
did u check the edge of the engine block for stamped numbers?
http://www.acmoc.org/bb/showthread.php?23474-Serial-numbers-on-the-edge-of-block
Looks like someone ground the paint off where the Serial # should be on the engine