I had no idea as my mother purchased prime rib all year round. I didn't know it was the same thing as a standing rib roast.
Two separate issues:
"standing rib" refers to the cut of meat.
"prime" refers to the USDA grade of beef.
Unfortunately, most people confuse the two and refer to any grade of rib roast as "prime-rib". Very few butchers actually carry USDA Grade Prime beef - is is exported to Japan or reserved for hotels and restaurants.
Butcher shops selling beef that has not crossed state lines often market non USDA graded beef.
The regular cow sold in markets is USDA Select, or it's equivalent The better cuts are USDA Choice.
der Brucer