Ok, so we know that Stan makes really silly and cartoonish doodles but what if he could also draw really fucking good? Like, yeah, sure, he was discouraged from making comics, but maybe he still sketched stuff he saw from time to time. Stan was definitely the type of kid to sketch the girl he's crushing on on his notebook during class.
Anyway, cut to post Gravity Falls. They're in the Stan o' War II, and ig Ford is looking for something and he goes to his and Stan's shared room. He sees scattered papers and pencils on Stan's desk and he assumes it's another Lil Stanley comic. Curious, Ford moves aside some papers to get a better look at the his brother's drawing, only to be met with a really impressive and detailed drawing of the Stanleymobile. And he just goes "holy fuck".