Continuing on.
The Brit Gun Trade built guns in shops ranging from "a single or couple of guys" to hundreds of workmen factories. Very few, even masters, developed the range and level of skills necessary to make a best work gun economically. Division of labor was widely practiced. Shops ordered work from smaller and larger shops to gain craftsmanship uneconomical to keep in-house. The trick was in obtaining commissions for best work guns; anyone could get one out (with the help of other shops and/or out-workers). Best work guns were NOT built by elves in quaint cottages by flickering candle light.
The Brit Trade guns are "honest" in that they almost always were built to be what they appeared to be. I've never encountered a factory "tarted up" Brit gun; aftermarket, yes, factory, no. That said, we must understand that there are exceptions to any rule you can make. We do not need to try to deal with all the possible exceptions, though. If you bump into something outside your experience or understanding, leave it alone, at least until you are able to research it adequately. Never take a "flier" hoping for a bargain - odds are much against you.
The current values for guns are set by the "market." The market has, over the years, decided what it will pay for the various names, grades, and conditions. My system has organized the market decisions into a more quickly learned form. The guys who handle thousands of guns a year develop intuition as to what a gun is and what it is worth. Unless you live near a major dealer, you will need to study auction sites, estimates, and realized prices.
Now, to the question. A best work gun will have 75% + coverage of high quality engraving, your first clue. The stock wood will be of high quality and usually well colored and figured - second clue. The stock will be shaped, checkered, and finished at a very high level of craftsmanship. Barrel striking and finish will be excellent. The action will also be well fit and finished. Unfortunately, there is no "short course" that will give you knowledge of all the above.
As grade goes down all of the above are subject to lower quality craftsmanship and/or materials. An "A" grade gun will have a slight reduction in engraving coverage and quality. Wood quality will often go down a notch, as will metal finish. A "C" or "D" grade gun may have no engraving, plain wood, a few "boo-boos" in the metal finish, etc. "B", "C", and "D" grade guns are often built on actions requiring less hand work and were often bought-in from the factories.
Questions to this point? More if anyone is interested.
DDA