It was always my understanding that the term denoted the best effort of the maker. It was not necessary that the embellishment be the most elaborate, but at a minimum it would denote the finest fit and finish in workmanship along with well executed engraving of at least a standard pattern such as Purdey's rose and scroll or Holland's "Royal" scroll.

For instance, a Holland & Holland "Royal" ranks as a best gun even though an H&H engraved by the Hunts would cost considerably more.

The makers often had lesser grades which were marketed as such. An example would be the H&H Dominion or the H&H Badminton.

My Purdey .500BPE is a "C" quality, which was about mid-grade, as I understand their grading. It is marked as such on the face of the breech and on the barrel flats.

Likewise, my Lancaster .280 Flanged is a "B" grade, and is so marked:

Curl