According to the write up at Fantasy Junction,
https://www.fantasyjunction.com/inventory/1964-ford-289-shelby-cobra/overview , the chassis was delivered to Shelby as a demonstration bare chassis which was subsequently given a fiberglass bucket T hot rod body for Shelby (which he refused to accept leaving the car at the contractor's shop).
The car was not given a Cobra body until the 1990's (created by a California shop). It was apparently at that point which it was "assigned" the COB 5998 ID.
So it appears to be a period, un-numbered(?), chassis with a replica aluminum body created in the mid 1990's.
John