The SM420 has a spring loaded reverse gate but it would be possible to add the lockout with some tinkering with the shift lever. You would need a pivoted plate mounted to the shift lever that prevents the shifter from moving into the reverse position by blocking its sideways movement. It might be necessary to modify the twist cap at the top of the transmission tower, also. Having both the spring and the lockout lever is sort of like wearing a belt and suspenders to keep your pants from falling down, isn't it?
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!