u/Bosco_stix

This chart accompanies a full article called "The Other Marty McFly?" by Bruce Gordon, published in the July 1986 issue of Starlog Magazine.

The article asserts that the two Martys live and grew up in separate dimensions and swapped places. You can read the full article here, but below is my summary.

We know what happens to "Our" Marty. He returns to 1985 in the Lone Pine timeline we see at the end of Part I. He effectively takes the place of the "other" Marty (Marty II) who was born and grew up in this timeline. The rest of the trilogy ensues.

But what happens to Marty II? We see him travel back to 1955 at the end of Part I, but we never see him again.

The theory claims that when Marty II makes that 1955 trip, his Delorean has extra plutonium stored. Why? Because his Doc is the same one who helped Our Marty return to the future 30 years before. Doc's knowledge of these events causes him to ensure plutonium is in the Delorean before the Libyans arrive. Marty II uses it to immediately travel back to 1985. Problem solved.

But there's a major side effect of this: Marty's intervention with his parents in 1955 never happens. As such, they become the versions of themselves we see at the beginning of Part I: Our Marty's original family.

So when Marty II makes the quick return to 1985, it's in Our Marty's dimension: the Twin Pine timeline. Marty II witnesses the time travel chase from afar -- the same one we see at the beginning of Part I. But since Our Marty's Doc isn't wearing a bulletproof vest, he is shot dead. Marty II proceeds to live in this dimension without Doc and unfamiliar versions of his family.

All aside, I think it's humbling to realize the fan theories we see on this sub were also being spun up 40 years ago, well before the internet. This is a super cool artifact for BTTF fans.