Prime rib does not always mean "USDA prime grade"

In beef, prime rib has long meant the best cut of the rib section.  The rib section is cut from the 6th to the 12th ribs, inclusive.  This means that the rib section does not include the 5th rib forward, which is part of the "chuck", and the 13th rib backwards, which is part of the "loin".  Iin meat departments across the USA you will often find "prime rib" roasts that include the loin section. See beef cut photo

A bone-in prime rib cut between the bones produces ribeye steaks.  Remove the bone and you have Spencer steaks.

As described below, chefs like Ranhofer in 1894 used the term "Prime Rib" many years before the USDA first adopted a tentative meat grading system in 1916. 

Ranhofer's reference

For example, Charles Ranhofer, the famous 19th Century chef de cuisine at Delmonico's Restaurant in Manhattan, explained the meaning of Prime Rib in his 1894 treatise, The Epicurean, at page 472.  Ranhofer's illustration of the American beef cuts shows three cuts, labeled A, B and C (with C being the front-most), which are described:

Six Prime ribs, A [11th and 12th ribs] first cut, B [9th and 10th ribs] second cut, C [7th and 8th ribs] third cut.  Id.

The 6th rib is also part of the rib section and can be used as a rib roast, but not a "Prime Rib".

USDA meat grading

The first tentative standards for grades of dressed beef were formulated in 1916, and the federal grading of beef began in 1927.

Conclusion

The term "Prime Rib" was in common use to mean the best part of the beef rib section, more than 25 years before the first use of the "Prime-Grade" designation by the USDA.

Related information

• Harris, J.J., H.R. Cross, and J.W. Savell (Department of Animal Science, Texas A&M University).  "History of Meat Grading in the United States".  November 30, 2001.