COPO is the well-worn acronym for Central Office Production Order. It was an internal Chevrolet business process for building a fleet of vehicles with non-production equipment. I believe the minimum order was 50 units; the customer proposal had to be reviewed by Chevrolet Engineering for approval and pricing. Probably 99% of the vehicles built under the COPO program were police cars, taxis, municipal and governmental units including trucks. Special paint and striping were also done under COPO orders.
Only Chevrolet built cars under a COPO order. Dealer modified cars are not COPOs.
Applying 'COPO' to virtually every car Chevrolet built with a 427 engine is inaccurate. The L-72 427 engine was an RPO for all 1966, 1968 and 1969 B-body cars. The L-88 was also an option for the 1967-69 Corvette as was the ZL-1 engine for 1969.
At this time the only '427' COPOs known are the 1969 L-72 Chevelles & Camaros and the 1969 ZL-1 Camaros.
http://www.camaros.org/copo.shtml