Call Jimi at Standard Transmission at 800-783-8726 or email him at
jimig@standardtransmission.com He can tell you identifiers on TKO500 vs 600.
I know one way - unscrew the shifter boot hold down ring (the chrome ring that holds down the rubber fitting at the base of the shifter) and look down onto the top of the tranny. 600's have a RED anodized plate where the shifter enters the housing (like Don's above), 500's are mill finish.
Jimi probably knows an easier way.