"The conceivability of being wrong" and "perspective-taking on beliefs" are old indeed; I wouldn't be the least bit surprised to find explicit precedent in Ancient Greece.

The most famous expression of this that I'm aware of originates with Lord Cromwell:

I beseech you, in the bowels of Christ, think it possible you may be mistaken.

Arguably, Socrates's claims of ignorance are a precursor, but they may stray dangerously close to anti-epistemology. I'm not a good enough classical scholar to identify anything closer.

The grid-visualization method / Litany of Tarski was invented on LW as far as I currently know.

The grid-visualization method seems like a relatively straightforward application of the normal-form game, with your beliefs as your play and the state of the world as your opponent's play. The advocacy to visualize it might come from LW, but actually applying game theory to life has a (somewhat) long and storied tradition.

[edit] I agree that doing it in response to a temptation to rationalize is probably new to LW; doing it in response to uncertainty in general isn't.