Hello.
The first digit is the year model.5 means 65,6 means 66,etc..The second digit is the plant that built the car.F is Dearborn,R is San Jose and T is Metuchan New Jersey.The 3rd and 4th digits are the body style.For 65/66 cars ,07 is a coupe,08 is a convertible and 09 is a fastback for 67 and up,01 is a coupe 02 is a fastback,03 is a convertible,04 is a Grande and 05 is a Mach1,except in 69 the Mach1s had the same 3rd and 4th digits as a non-Mach fastback.The 5th digit is the engine code.For the 64 1/2s U is a 170 6cyl,F is a 260 2bbl,D is a 289 4bbl and K is the hipo 289 4bbl.65-67 T is a 200 6 cyl,C is a 289 2bbl,A is a 289 4bbl, K is still the hipo 289 4bbl,and S is a 390 4bbl.In 68 they added F for a 302 2bbl and J for a 302 4bbl and phased out the 289 entirely.67 was the last year for the K.In 69 they added L,a 250 6cyl.In late 68 they added Q,the 428 4bbl and R,the ram air version of the 428 4bbl.In 69 they also added the M,a 351 4bbl and H,and 351 2bbl,G is a Boss 302 and Z is a Boss 429.The last 6 digits are the sequential production number,with the first one of the year model being 100001.Hope that helps.