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.