@DoubleAA re this question, here's what I see at HebrewBooks:

And here's what I see at e-daf; I've marked the last line from HB:

@MonicaCellio The part on the bottom of the e-daf version is the commentary of Rabbeinu Gershom, which is on the right side margin in the HebrewBooks version. It doesn't look like there's actually any additional content; it seems to be just arranged differently.

@Alex oh! Thank you; I totally missed that it was just formatting.

Ok, I've deleted all those comments arising from my mistake.

@MonicaCellio No problem.

There's the commentary of a rishon or gaon or sometimes acharon in the outside margin of most (all?) masechtos. I've no idea since when (has it been there since the original Vilna Shas??), nor who decided which maseches gets which commentary, nor why. Some that come to mind besides Rabenu Gershom above are Rabenu Chanan'el, Tos’fos Ri Hazaken, Shita M'kubetzes, and Tos'fos Y'shanim. There are undoubtedly more that I'm not thinking of (or don't know of).

@MonicaCellio ^

@msh210 thanks.

@MonicaCellio Pleasure.

@msh210 It's described on the opening page that's printed in most reprintings of the Vilna Talmud - mostly they're manuscripts that were first published as part of the Vilna Talmud, so apparently they were just printed on whichever tractates happened to show up.