Bigcy, the link I posted was a poor example, but yes, if these were mounted on a steel rod (as they sometimes were, rather than brass rod. Our example is steel rod) they could be easily "lost". Not lost by falling off , but lost to accident. Often the carb is exposed to weather, as are the valve stems, rocker arms, etc., and sometimes the intake can sieze. Another common problem is the spring itself breaking in which case it has to be taken apart and replaced. It can easily break in the field doing these things if you try to pry it out or bang it a little two hard.
The other example found on the beach in another post is brass mounted. It appears to have been melted rather than corroded, my guess being an engine fire.
Again, these are guesses as to what it is. Although it is an exact match for some intake disk valves I've seen, with perfect camphoring, it could as well be a check vlave of some sort for something else that didn't need to be water tight that I'm not aware of.