Information concerning D.S.O.'s has been revealed by the SAAC registry research on 1968-69 Hertz rental cars. They "acquired" some official Hertz documentation that partially unravels the D.S. O. mystery. Apparently the last four digits of a D.S.O. is tied to a special order invoice. Note that 1969 Hertz Shelbys were made in two different consecutive unit number batches, but were from the same special order invoice. My speculation is that the Ford production line had their own reasons, such as filling "regular" retail sales orders from dealers, so as to keep up with dealer demand while satisfying the large (large for a special order request) Hertz order. Therefore if the 1966 HCS cars had, for example, three different D.S.O. numbers, then the Denver office made three different and separate special order invoice requests. They probably ordered a batch, sold them all, then ordered another, etc. To further prove that the last four digits are tied to a special order invoice request, a few replacement Hertz Shelby's ordered in 1969 had a different D.S.O., because it was a separate order. Finally, when we think we have most of it figured out, there is an exception, at least one Shelby with the Hertz special order D.S.O., was sold as a "retail" sale.