Which patch specifically is run in this category? It's not listed in the rules or anything.

Looking at the speedruns, it seems 1.0.0.5a.

At the moment, it's been whatever patch isn't current and is also the fastest. I can elaborate on the rules, but I've kept them pretty sparse since the game is new and everyone is still figuring out how they want to run the game.

As for the specific patch to run, some bugs were fixed in 1.0.0.4, like the ability to confuse the game into letting you equip insane amounts of ammunition all at once -- check out Andy's new old patch WR to see that sort of thing in action.

Ty for the clarity :)