I've got a question about the vent window assembly for a 69 Road Runner/B body. If you look at the attached picture at the left at the very bottom of the channel for the door glass there is metal bracket that comes off of the channel that has a round hole. I've seen about 50% of the vent window assemblies that have this bracket and 50% that don't. What is it for? Is the piece just missing on some of the assemblies that I've seen or do some cars just not have them?

I'd appreciate if someone could clear this up for me. Thank you!