Look into the history of cars, and you’d find that many models on the road today do not carry the same form factor as their originals. The Honda Accord, for example, began life in the 1970s as a three-door hatchback. The first-generation BMW 3 Series only had two doors, but there has not been such a thing as a two-door 3 Series since 2012.
This brings us then, to the BMW 6 Series, which began life in 1976 to replace the E9 CS coupes. The original 6 Series, codenamed E24 and a highly-collectible classic today, sustained a 13-year production run till 1989.
Cosmetically, the E24 wore the shark nose front of the 7 Series, but its mechanical underpinnings was based, initially, on the E12 5 Series until 1982, when BMW re-engineered the car from ground up to sit on the E28 5 Series platform but kept the looks unchanged. A highly-uncommon practice.