'69 Barracuda Driver's Side Door Window (Pics included)

I have 2 driver's door windows for the '69 Barracuda. One has a knob (lack of a better word) located in the lower left of the window and the other does not. Which one is correct? The part I am questioning is item 77 & 78. Does anyone have an idea of what this is for and if it is convertible specific?

Thanks,
Tom
I will have to dig through some old notes on it to be sure I am 100% remembering correctly but that is specific to 69 and i believe is some kind of up stop. While the glass from 67 to 69 is shaped the same the track was changed in 69 and makes the glass 69 specific. Like I said I know I have some notes on the details, I just need to dig them up. It it gets too hard finding just the 69 glass you can always change track and all with one of the older ones to get yourself operational.