'65 Formula S rear springs

According to 440source.com (see quote below), both part numbers mentioned in post #1 made their debut in 1964. They may have been used in any number of model years after that date.

"Now on to the numbers themselves: Chrysler's part numbers are date specific, and if the numbers are broken down, it is possible to determine the year that the part was first put into use. Note that the number only tells you the FIRST year it was used. If the part continued to be used for five years, the number will not indicate this. The numbers will almost always be a 7 digit number. For clarity and ease of writing, this seven digit number is usually broken up into two parts, a prefix and a suffix, similar to a seven digit phone number. However, the way it is split up is with the beginning 4 digits first (the prefix) and then the last three digits (the suffix) which is a mirror image of how a phone number is divided.

A typical part number might be a cylinder head: 2843904. The first digit represents the decade. A "2" in this case is the 1960's. Part numbers starting with a "1" would be from the fifties. The second number represents the year. So in this case "8" would be 1968. This is why most big block parts will never start with a prefix earlier than 17 or 18, as they were first designed and released in late 1957 for the 1958 model year. This date system holds true for almost all part and casting numbers up until about 1969.

During 1970 for some reason (except for a few 30XX parts) the 30XX-33XX prefix numbers were skipped and part numbers started at about 34XX, so most numbers for 1970 and 71 will begin with 34 or 35. An example would be part number for a 1971 440 4 barrel intake manifold: 3512 502. The 36XX prefix started in late 1971 and was used through most of 1972. 37XX started in late 1972 and was used through most of 1973 and 1974. 38XX started in late 1974 and was used mostly in 1975. 39XX was very limited and used in 1975, and then the numbers rolled over to 4000 prefix. One famous part that began to be used in late 1975 is the 4006 452 heads. Numbers seemed to stay in the 40XX and on a more limited basis, the 41XX prefix through all of 1976-77-78 to the end of production."