The 666,501 figure agrees with the Production Figures page on this site. It also shows Riviera production at 37,336.
Per the Chassis Service Manual, Riviera car serial numbers started at 900,001 that year. At least with that numbering system, it is easy for a Riviera owner to tell where the car fell in the production run.
On occasion Buick would assign car serial numbers by car series, assembly plant, or engine type from irregular blocks of numbers.