Not sure when they changed from snaps to plastic tab retainers, but I can vouch for 1971-76 big car convertibles have plastic tabs that slip under the pinchweld trim moulding, from the rear.
When you say "parade boot" do you mean a soft vinyl boot, or a hard plastic or fiberglass cover? Most people assume a parade boot to be a hard cover, which was generally available only on higher line full size cars.
It's possible you have an aftermarket boot which may have attached differently than the factory piece.
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Glenn Williamson
Rocketraider
member AACA and all major Olds clubs
1964 Starfire, 1969 Toronado, 1974 Hurst/Olds, 1976 Ninety Eight
There's a Dodge K-Car and a Ford truck hiding in there too! Hey, ya gotta have something to beat around in...