[Jim Maxwell]

The original Cobras were identified on paperwork from the factory with the letters CSX and a four digit number. The 2000 series of numbers were for the leaf-spring cars (260 and 289 cars) and the 3000 series of numbers for the coil spring cars (427-428 cars). The four digit numbers were stamped into various parts of the car, or scratched into aluminum panels of the car with an electric engraving tool. The last 3 digits of the 4 digit number were stamped into other parts of the car, like the hood and trunk latches.
The letters CSX applied to Cobras sent to the Shelby factory for the US market. There were also COB cars which were for the British market (right hand drive).
Most of the original cars had a serial number plate pop riveted to the right hand (passenger side) foot box with the serial number stamped or engraved on it. The 289 cars also had the serial number of the original engine block stamped on the tag. These cars had "matching number blocks" like Corvettes! The 427-428 cars did not record a block number.
Atleast for my cars, these numbers with the CSX prefix are the VIN numbers on the titles. Needless to say, when you try to register one of these, the DMV doesn't like VINs that short. Early Ferraris are even worse, some of them with only 3, 4, or 5, digit VINs, and no prefix letters at all.