When GM's full-size cars were redesigned for 1971, the Eldorado regained both a convertible model and its fender skirts. The 126.3-inch (3,210 mm) wheelbase version of the Eldorado would run through 1978, receiving facelifts in 1973 and 1975. The Eldorado, Riviera and Toronado shared the redesigned E-body chassis. However, as before, Eldorado and Toronado were front-wheel drive, while the Riviera (which underwent three design changes in the same timeframe, including a temporary switch to the B-body platform for 1977) was rear-wheel drive. Sales set a new record at 27,368.
Although the door glass on all Eldorados remained frameless, the hardtop rear quarter windows vanished, replaced by a fixed "opera window" in the widened "C" pillar. This was likely done to conform to threatened Federal rollover safety standards that were scheduled to take effect by the mid-1970s.
Source: Wikipedia, 2012