No, that's the earlier type "crash box" non-synchronized transmission. In that box, the gears are actually moved in and out of mesh with each other instead of staying meshed all the time and synchronizers doing the actual engage/disengage functions. The only non-synchronized gear in a SM-420 is the 1st.-reverse sliding gear. (granny gear) Chances are that the sliding rail in the shift tower that engages reverse is stuck or rusted into the neutral position.
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!