The official first day of Mustang mass production is widely recognized as Monday, March 9, 1964. All Mustangs made on this day have a date code of 09C (09=day and C=month). Units made on March 10 had a date code of 10C, and so on and so on. The lowest known 09C date code (March 9) consecutive unit number is 100211, which is a six-cylinder hardtop. (Note: numbering system started at 100001 )
Prior to the March 9 launch date, Ford built approximately 180-200 pre-production Mustang units between January and March of 1964. Each of these units were labeled with a date code of 05C. The 05C date code does not mean March 5, but instead was used to identify all pre-production units built before March 9, 1964. Consecutive unit numbers for these "05C" Mustangs run from 100001 to approximately 100180, with the highest documented number being 100178. The reason for the gap between 100178 and 100211 is still unknown at this time.
Pre-production Mustangs were produced for a variety of purposes including promotional cars for the 1964 Worlds Fair and dealer advertising, press releases, racing pace vehicles and actual race cars, engineering purposes, executive employee use and even crash testing (ouch!).
Pre-production units were used as a way to establish assembly procedures and make engineering revisions. As the manufacturing standards had not yet been set, the assembly of the early pre-production units used many hand-made parts that had to be modified on the fly. This modification process, sometimes using a hammer or torch, is more prevalent in the earlier pre-production units. Latter pre-production units incorporated the earlier changes and more closely resembled the mass production units.
No, this is definitely not an exact science. There are additional hand-written numbers on the passenger and driver side of the front radiator support, which represent inventory control, rotational and job numbers. These numbers, in conjunction with the quantity and type of "irregularities" identified, will help give us a better chronological understanding of this Mustang.
There are approximately 15 that have been verified and known to exist world-wide. Ford crash tested and sent many of the pre-production units to the crusher. It is my personal belief that they never intended for many of these units to be made available to the public.