The Big 3 were sorta in a state of upheaval in the late Fifties-early Sixties. GM had a "One-year Only" body, that really was a 2 year body, in 1957-58. Some of their marques, basically the higher-end models got a new body in 1957, but the lower-priced ones had to wait til 1958. But in 1959, they did it AGAIN, EVERYBODY had a new body in '59,& kept it til 1961. In the case of the Cadillac 75, their Limo, it stayed w/the 1959 architecture thru the 1965 model year, even tho all the rest of the line got totally new bodies in 1965. The OUTWARD sheetmetal, the lower body panels were changed, & kept reasonably current. The Top/roof sheetmetal, is always a lot more complicated, & typically doesn't get changed as often as do fenders, hoods, deck lids. The 1966 75s stayed pretty much the same thru the 1970 model year, & then they changed w/the rest of the line in 1971, & kept that design thru the end of the "Traditional" big cars in 1976.