The sequence number represents when a car was ordered. At that time it was immediately assigned a VIN with a sequence number. The scheduled production date very often did not align with the sequence number. So a Comet with a sequence number of 805678 may have actually been produced and ready for shipping before a Comet with a sequence number of 805432. The sequence number should be considered to be a unique identification that shows when the order was received and when it was planned for production.

1960-1969 Mercury, except Comet sequence numbers started with 500001

1960 and 1961 Comet sequence numbers started with 800001
(The Comet was a stand-alone make those two years...the word Mercury does not appear anywhere on those two years of the Comet.)

1962 - 1969 Comet merged with other Mercury cars so their sequence numbers begin with 500001