Auto Enthusiast feb 2012 issue has an article about the 1966 "Carryover" Shelby GT 350 Mustangs

I had never heard of them, but there were 252 66 GT 350s that started life as 1965 Mustangs, and were ordered to prepare for the stoppage of the 1965 production and retoolling time before Ford started building the 1966 Mustangs

So Shelby had to order 250 cars to have on hand for his companies conversions to GT 350s while Ford would be unable to keep up the supply. This way when Ford started selling 1966 Mustangs, Shelby would have 1966 GT350s

2 cars were prototypes (6S001 and 6S023)

For more about the "Carryover" Cobra Mustangs http://www.carryovergt350.com/