There’s always been a bit of uncertainty about the identity of the earliest Mustangs, as Ford never kept exact production records and the earliest cars weren’t built in sequential order.
An original Ford Mustang claimed to be the first hard-top example is coming up for auction. It will go under the hammer at Mecum’s Indy 2017 auction in mid-May.
Check out this 1965 Mustang with VIN ending in 00002, meaning it comes after the 1965 Mustang convertible with VIN ending in 00001 currently sitting in The Henry Ford Museum in Dearborn, Michigan. Both of these were pre-production examples. (the famous 1964½ model year was only unofficially applied after sales started to differentiate the earliest '65 Mustangs from later versions that received some updates.)
It is safe to say that this the first Mustang coupe, given its early VIN and numerous features that only pre-production Mustangs came with, such as the straight shift lever and prototype sheetmetal stampings and welds. Ford has stated that this is the first Mustang coupe.