You have to remember that GB is not as robust as Google. Searching in Double Shotguns or even Guns and Firearms doesn't yield the same result as Collectible Firearms. Additionally, searching for Parker Brothers doesn't at all give the same results as Parker. The Parker entry will turn up a lot of useless leads (parkerized) but many (most?) listings for these guns simply use the term Parker, not Parker Brothers. I just did a quick check putting the two different searches in and based on that, you miss the bulk of the Parker gun listings by searching on Parker Brothers. I search daily for "Parker" in both "Guns and Firearms" and "Collectible Firearms". It takes a lot longer to sift through, but you can examine the appropriate subheadings and get there.

If you don't do this, you will not see a huge number of the listings.