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The different C designations are usually determined by a redesigned body/chassis design that can last for years with subtle and sometimes extreme body skin changes. C1s 53-62 are all basically the same cars under the bodies of these straight axle cars. Sure there are ongoing updates and improvements through those years but the "bones" are the same.
Same for C2s as this was a dramatic change from C1s with a new suspension that included rear independent suspension and a complete new body design.
C3s were basically just a new body design (pretty much the same "bones" as a C2 until some serious changes in 1984 (C4) but even then most of the changes to the chassis/suspension was lighter materials used. This C3 body style lasted 16 years. If you put a 1982 next to a 1968 they don't resemble one another much but they are the same basic car with simple body skin design changes.
You can follow this same scenario to present day.