The Royale interior was available only on Ninety Eight Luxury Sedans. Those are Pontiac A-body seats. It's possible, but improbable, that the assembly plant goofed and installed the Pontiac gut especially if it came from a Buick-Olds-Pontiac assembly plant like Atlanta or Arlington, but it would have been a sloppy dealer indeed who let it get past new car inspection. Or as you suggested it could have been done at the dealership after delivery, so there may be a GTO owner owner out there going
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I ordered a 1984 TransAM (only way to get the 5-speed, and even then it took forever) and it came in with one TransAM fender and one Camaro fender, complete with Camaro script and sans TransAM vent. I wouldn't accept the car, reasoning that if they were that sloppy assembling it, it was likely to be a lemon, and I wasn't about to have a new car that had to have major bodywork before I even took possession of it. Salesman told me later that the car had created a big stink in the Pontiac sales organization, especially since it was a customer order instead of a dealer stock order.
So yes, factory goofs did happen.