COPO (or special order as you call it) allowed a person to order any combination of anything, with or without anything.
Trouble is the factory didn't offer COPO to every person who wanted to order a car. Fifty or more (as in a power or phone company wanting a fleet of vehicles to be equipped their way with a custom mixed paint) that wasn't a problem. But you wanting a single body in white with an L-88 and a deluxe interior to go in a carry out box was a different ball of wax.
The district sales manager usually got involved in COPO orders so there was usually a trail of paper that could have been verified if anyone had kept that pile of paper; but you must ask yourself have you kept last weeks grocery receipts? To GM's central office a COPO had the same value as your cash register receipt for a summer cook out. Nadda
Most people think COPO is some kind of holly grail, it isn't it is just an order generated by any commercial customer desiring a vehicle built their way. Rail road companies, cities, utilities, all of them had access to COPO to order anything they wanted, and still do.
My father bought a Caprice Classic Deluxe with a full 9C1 police package on a 1968 COPO car that the city of Clearwater Florida had ordered and built for the chief of police. The reason my father bought it was because the city auditor didn't want to pay the bill for the COPO car as being to luxurious. It was outside the amount authorized per car by the city council. The dealer was stuck with it (you can not return a COPO, you order it and it is yours) and since it had a 350 horse 327 under the hood, TH400 tranny, 3.55 12 bolt with four wheel disc brakes; not to many old ladies were in the market for it, so my dad got it for cost. In the records of Impala SS cars built there is no mention of this car, though it was an SS in terms of suspension, power, cooling, batteries; by any definition accept badges it was an SS.
Big Dave