I'd say Dana's correct, that's a grounding style holder, going back to the 1st pics. There would be the same terminal retention hole in the tab as the other brush if it were meant to have one attached , and the holder body is narrow, so even tho' We can't see the insulating ring's presence I'd say it's not there. Get the '73 brush kit... it's really poor form to manufacture a dedicated grounding brush with a tab large enough to actually attach a terminal to, what could go wrong there???...oh, YOU found out, glad Your car didn't catch fire.