Greg,
I’m no expert but if there is one thing I trust when buying meat it’s the reputation of the establishment and if they say it is prime and charge a premium price compared to their choice grade then I’m going to believe them! Most all meat today are sub-primal cuts from one of the few national firms where the meat is graded. Could it be corrupt? sure…but they won’t be doing that for long! When you mentioned what you saw in the trimming last night that was the lateral(horizontal) slices which really wouldn’t reveal prime vs choice. Instead its the vertical cuts where you see the face of the steaks. Notice the fine marbling in these two pieces.
What I do when selecting a sub-primal to age is to inspect the ends of the cryovac as well as I can looking for signs of the marbling. You can do the same at Costco – just ask them to show you several to choose from and most will be happy to let you look!
Ron