No they are not Knock offs , nor were they using factory left overs. They are period correct for a '67 Ambulance depending on the Coachbuilder.
Cadillac did not make ambulances, coach builders did. Cadillac and the big 3 provided the commercial chassis, front clips and rear quarter panels only. the rest of the ambulance was scratch fabricated by Coach builders such as Miller-Meteor, Superior, and Wayne to name a few. Coach builders used whatever was available at the time to create there distinctive looks,styles and to streamline the process, not to mention save on costs. The 1959 cadillac tailight was very popular due to its unique design and was used for many years on various ambulances as a warning light. Not to mention its popularity with hot rodders and customizers as well. The 59 cadillac taillight would be attached to the body or onto the end of a custom made tunnel light.
coach builders used many different taillights of various makes and models to achieve their desired look. all these pictures feature tailights as warning lights on ambulances, including late 60's Pontiacs with with 59 caddy lights as warning lights.
So yes someone did make thier own, the Coach builders did .......and now you know!
In no way are they worth $350. They are worth the contents of thier parts. 59 cadillac taillights are still being made today and are abundant, light sockets, flasher, cable and brackets. Sorry to burst that bubble but it is fact. Now if they were attached to some custom made tunnel lights, that is a whole diffrent story and price range.