The straight-style was used until 1979 for a 3 1/4 inch hole and just snaps in place. Many aftermarket windshield makers drill 3 1/4" holes in their windshields for the straight-style vent. The angle-style was later created for a 3 3/16 inch hole.
To replace angle-style vents with these straight vents, use a 3-inch sanding for your drill and enlarge the holes 1/16 inch to make the straight vents fit.
Another method is to use a drinking glass with a little taper and correct diameter. Wrap the glass with sand paper and enlarge the hole by twirling the glass and sanding until the new vent fits. This method will enlarge the hole and make it perfectly round.