the wiki page for PHGP says that Billie wrote it under the name Wilhelm Fink, as well as the songs 27 and New Blood. 27 came out in 99 when Billie was 27, so that lines up, but i always thought it was a pretty mediocre song, definitely not his best. new blood is an ok song though, but i think Anniversary song is the best one he wrote for Pinhead Gunpowder. so yeah, he's credited as playing on all those albums as Billie, but for some reason the songs he wrote were credited as W Fink, but then the majority of the rest were music by PHGP and lyrics by Aaron, but that doesn't really solve the mystery as to why he used that name. maybe it was a legal thing with reprise