Imho it should be either ref:gvp (or ref:vn). Both are KISS and not currently in use for something else. If there are multple volcano databases having a “Volcano Number” then ref:vn is a no go.
“SI” didn’t resonate with me at first. The word has exactly 1 =archaelogical_siteref:smithsonian | Keys | OpenStreetMap Taginfo
But neither ref:Smithsonian:*= nor *:GVP= covers it all. The database is VOTW.