You need to read the 1955 Shop Manual 2nd Series, section 1-7 covers the vent window.
To remove the vent you first need to remove the roll up window, then find the 4 (I think) screws along the top edge of the door after the garnish molding is removed, then remove the two screws in the middle of the door panel at the bottom of the divider and then remove the entire divider with the vent assembly attached...you do NOT need to drill out ANY rivets. I might have missed a step or two, but this will get you close...If I remember corectly it takes a little twisting and wrangling to pull it out of the door as there are tabs/flanges making it a little tricky to lift it straight up and out.
This is what it will look like when removed...
https://www.classicindustries.com/p...8vGe2EfyNgaArlqEALw_wcB&gclsrc=aw.dsMike B
