The box in your photo is not an SM-420. It's the old crash box non-synchronized 4 speed, which is the only one with a reverse lockout. It will also have a different bellhousing bolt pattern, so a 420 will not bolt up without swapping to a 1948 or later bellhousing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!