History And Usage Of Usir "Silver Tax" Stamps

I recently came across this USIR 2-cent "Silver Tax" stamp. Based on a quick Google search, I think that it is Scott #RG2. (I had mistakenly filed it as stamp R229.) At the risk of showing my naïveté, I wasn't previously aware of Silver Tax stamps (in part because there doesn't seem to be a "Silver Tax" section in my Scott National album, and in part because the dealer that I usually work with doesn't show them on their web page). I am curious about "Silver Tax" stamps and how they were used - can anyone provide a brief primer? Are there also USIR stamps for other precious metals (e.g., "Gold Tax" stamps?) that I simply haven't seen?
Thanks in advance for any input on this topic.

No gold tax. It was really a silver purchase tax of 50%. As might be expected, KODAK was the biggest purchaser; there are documents around. There are also some very rare provisional usages. Looking at the PF site might be instructive.

Thanks, revcollector!
Sorry to ask, but what is "the PF site"?

The Philatelic Foundation. philatelic foundation.org. You can search the stamps which have certificates. Search either RG or at the bottom you can search silver tax.