I stole the sig from Metis Highland, founder of Triunism. One or two other Triunists use that signoff too. Metis took it from The Dark Tower, I have since learned, but in BM it's kind of a Triunist thing. Since Hireshmont's conversion to Sanguis Astroism, he's actually gotten a sort of a semi-cease-and-desist request from one Triunist, as well as a few Astroists asking him if he's "really" converted, given his signature.
Well thats a bummer because I was thinking about stealing that awesome signoff but if I will get in trouble with the religious folks I probably shouldn't.