Okay, here is the deal-io. Being a noob on this board, I wanted to check my bike and be sure before I posted.
Having just gone through this procedure to move the switch back to it's original and rightful location under the tank (PO had mounted the switch up by the instrument panel), I was thinking it was a tabbed washer.
Just checked my '76K - indeed it is a tabbed washer - on my bike at least. The washer has two tabs, one goes in the "female" tab for the mounting bracket. The other locks into the recess on the ignition switch itself.
I got an original mounting bracket off ebay from a '74 (IIRC). I guess I was lucky because mine came with the ignition switch, mounting screw, key, and... the tabbed washer.
Check ebay, I'm sure you can find one. You may have to buy the switch in order to get the tabbed washer. I have a picked up a lot of "extra" parts this way.
![Grin ;D](http://forums.sohc4.net/Smileys/default/grin.gif)
Cheers!