Looking at Sirectifier's website, and looking at apparent patterns in their product listings and part numbers, I'd go out on a limb and bet that the 12 means "1200V". Would I bet my life on it? Hmmm... not sure. But a few dollars anyway.
Maybe try contacting the company? sales@sirectifier.com shows up on the website. Who knows, maybe you get lucky and get somebody friendly on the first try and they give you the exact info you need? I mean... that's gotta happen every once in a while, right?