It's true that the Yellow doesn't have much vintage content, and tends to focus more on the modern airguns, but the sheer number of airgunners on that forum makes it an important resource. Several of the regulars on the Yellow are experts in vintage US airguns, I think, and don't post elsewhere.
But as you have discovered, the real experts on vintage US airguns can be found on the American Vintage Airguns forum. Some of its regulars are experts on European airguns too.