User blog comment:GrilledMeatling/Gravity Falls Theory: Who wrote the Books 3, 2, & 1/@comment-25259727-20140803092018/@comment-25268537-20140805135252

Another piece of evidence to add from that episode, the computer they found says "Property of F". Old Man Mcgucket's first name is Fiddlehort, which means the computer could be his.