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Aston Martin did something similar before with the DB4 Zagato. They had an extra 4 chassis numbers that had never been used, so they built 4 more of them. Perhaps them found some DB5 chassis numbers that were not used. From Wikipedia:

In 1988, four unutilised chassis numbers were put to use. With the approval of Aston Martin, four DB4 chassis were appropriately uprated to GT specifications. These chassis were then sent to Zagato's Milan workshop to be bodied like the originals, with a smaller oval grille, without the stock DB4 GT's rear tail fins, and with a smoothed out rear end. To familiarise the workforce with construction techniques of the 60s, an original DB4 GT Zagato was sent there to be dismantled. These "Works-Approved Replicas" were known as the "Sanction II" cars. They were outwardly identical, but several changes were effected in the interest of better handling. Each of these cars sold for over $1,000,000. Differences from the "originals" include a larger engine capacity, increased from 3.7 to 4.2 litres, and a smaller wheel diameter, reduced from 16 inches to 15. The first of the four GT-specification rolling chassis was delivered to Zagato in January 1989 and the fourth in April of the same year, with all four being completed in July 1991. All four cars were then given their own chassis numbers, fitting for the 1960s.