Could more stamps show up? I would say yes and I would bet there are many of them sitting in a few family bibles in the homes of non stamp collectors.
Just think if another Z grill(85A) would show up. What would the market place do if a few dozen were to come out?
There was a guy just a few years back that was making more than enough grill stamps and selling them on
ebay. For a knowledgeable collector the difference between those and the real ones were easy to figure out.
A more interesting grouping of stamps are the 1975 reissue of the 1961-66 regular issues. Just try finding unused blocks of these stamps. They are in the market place but they are considered great rarities.
I don't know just give me a Scott number with a dash (-) in place of the current value then I just might have my retirement money.