Best source for researching the company and the wrench would be Google Books.
Value of obscure specialty wrenches is always difficult to assess due to lack of precedence. Rarity does not always mean valuable. It could be the only one in the world and worthless. That's the Catch-22 of obscurity. On the other hand, there could be some niche guy out there with an antique firefighting collection who fancies it. Without a corpus of exchanges, though, or even a nexus of discussion in a collecting community like this, the value is very unpredictable.
In general, most farm implements, and wrenches from the gas/burner, hydrant, oxygen tank, etc trades are not as popular or as desirable as automotive.
One interesting feature of the wrench is the way the markings were applied. The raised lettering may at first glance appear to be forged or cast in, when the wrench was made, but it is not. It is on plates that were patch-welded onto the wrench after it was forged or cast. This is sometimes seen with model number changes on vises, for example, or C clamps where the mfgr took the more economical approach instead of updating their dies.
It could mean that Foamite Childs' OEM was cheap, or it could mean they were making the same wrench for multiple customers. My hunch is the latter.
