On the SM-319 the lockout rod is moved by the 1st.-reverse shift fork as it goes into the R position. The rod is spring loaded into the "released" position so it disengages when the transmission comes out of reverse. The Saginaw most likely works in a similar fashion.
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!