I have a feeling that S1 cars were built of stronger stuff by workers who were proud.
At least they were prepared and protected better than S2's,,,
Then with the coming of BL things went downhill rapidly.
More so on new models coming online in the early 70's than the P6 line,but when the SD1 came along,corners were cut,cheap materials used,and the tail end of P6 production they probably suffered quality issues,like paint.
My uncle had a new 2000tc in about 1970 and every year would pop the wings off,clean and protect the base unit,it was still solid the day he sold it in 1975/6 and bought a new 2200tc.
He did the same with that,but it suffered what is commonly called inside-outside rust,in other words you dont see it till its a hole!
He got rid in the early 80's,and bought a new Jag XJ6 instead as he was then in his late 70's,that just about lasted him till he gave up driving in 1991.