I found that briefly very disturbing -- it seemed that the mere presence of an "L" or "R" on the vertical leg was sufficient to get my brain to see it one way or the other, even though I'd given no conscious thought to the question of which leg was which.

But of course it isn't actually the choice of letter that makes the difference. (I haven't said what is in case anyone else is in the same boat as me and wants to take a few seconds to work it out.)