Data analysis of a whole bunch of auctions showed me that Brit and Continental 12 bore SXS guns of 1880ish to 1960ish vintage value at USA retail based on Brand Value level (BV), Original Quality grade (OQ), and Current Condition level (CC).
"Guild" (nameless or very little known names) guns have BV5 (basically no brand value) and value at 1/8 of a similar BV1 (Boss, H&H, Purdey, or Woodward). This says that the market attaches a great deal of value to the name on the gun, like it or not. The old saw of, "Buy the gun, not the name," rings true for a gun to use, based on quality and condition. However, you will pay for the name, or not if there is a lack there of.
