So what's the difference
 

... between the 74-75 turn signal switch 911-613-305-00
and the 76-89 turn signal switch 911-613-305-01.
I need to replace mine (high beams come on when I pull the lever, but wont come one when I push the lever.)
Seems both styles are available on ebay from time to time.
I'm guessing they're mechanically the same, but something must have changed.

TIA

That is a serious wear part. I'd buy a new one. Wayne sells them right here on this site.
Not exactly sure, but I think the handles a little different.
You can go in and tinker with the contacts. But it sounds like yours is worn out.
My .o2c

You can get at those contacts with a fingernail file. Steering wheel and plastic pieces must be removed. I've 'fixed' mine twice. The second time, I installed a relay kit and I don't expect any further problems. The reason they fail is because all that voltage is going through the switch. The relay kits solves that design problem.

Hello

-75 has a slim stalk

76 on the paddle style end

often the plastic slider that moves the contacts is melted beyond repair, because of the current draw of 100W lights, which need to be relayed.