So, just to update a few things.

We also know the author's first initial is F. Old Man McGucket's first name is Fiddleford.

Also, while I saw a left hand on the front of the journals, my brother Akano pointed out that the symbol is just an outline and that there is no way to tell if the palm of the hand is facing the cover or the journal or not. Basically this means that the symbol could depict either hand, right or left.

How can we find proof? The six-fingered glove Grunkle Stan put on was for his right hand. Those types of gloves normally aren't ambidexterous. We don't know for sure whether the author only had six fingers on one hand or not, but if it was only one hand, my money of where the extra finger is located is the right hand. Old Man McGucket has his right hand bandaged to his knuckles, and when viewed from the side it looks completely covered along the side.

Theory: The author lost his extra finger somehow and then wrote in the journals. I have a feeling Bill had something to do with it. Proof of this is the bloodstains in journal 3 on Bill Cipher's page.

Granted, none of this evidence proves without a shadow of a doubt that Old Man McGucket is the author, but at this point I'd say it's somewhat likely. The identity of the author could still be someone we have yet been introduced to.

Also, more on the identity of Stan's sibling. Dipper, Mabel and Stan all share the last name Pines. This means his sibling was likely male as Stan is from an older generation where typically the man's last name became the family name. If this is true, then the twin theory seems to hold some merit. It's possible. Some people think the young Stan poking his head out of the Mystery Shack was actually his twin brother (in Time Traveler's Pig). It's interesting.

Another recurring theme in Gravity Falls is the year 1982. This takes place in the summer of 2012 meaning that 1982 was thirty years ago. The old room in the shack that held the body-swapping rug was not touched in many years. Behind the door, there is a calendar of July 1982 present. My theory is that the Shack belonged to the author previously. What's also interesting is that below the calendar on an end table, there is a pair of glasses. These glasses are on Bill's wheel. The author's notes on the wheel have these glasses circled with three exclaimation points pointing at them.

I don't think these represent the author, as the six-fingered hand was also on that wheel. In fact, if he'd just recognized the symbol, then why not point those exclamation marks to his own symbol as well? I think the glasses belong to someone the author knew personally, someone he knew well enough that they would leave their glasses in his home. Someone he wasn't expecting to be involved in all of the weirdness of Gravity Falls. I think this might play a role in three words that were written down in the journal.

Trust no one.

MUAHAHAHAHA!

~Melon Lord