I have to make a correction here. This particular casting number came up in previous threads, one of which I responded to and had forgotten.
See here. The consensus is that, despite what the web page I linked to shows, this casting number is from the SM420's predecessor, which is the '31-'47 non-synchro 4-speed.
To date the tranny, you can look for the case casting date code (a letter followed by 2 or 3 numbers cast into the case), and/or the tranny serial number (stamped onto the top rear end of the case).
I'm not sure about parts availability for these old pre-SM420 transmissions.
Is this tranny out of your '40? If so, it could be the original.
