Okay, I think I've figured out what's going on here. As usual, this is basically mostly my bad.
The number 391 converts to 1451. So you are partially correct in that it's supposed to be a 5 instead of a 6. As for the 2, the cross inside the circle is supposed to be a 4, and the diagonal dash inside the circle is a 2 (note that the two is not a diagonal on the actual puzzle input panel but is a straight line). I can't fault you for getting that confused, because on the poster in question, not only is the 2 slightly different than it is elsewhere, but the numeral that is used for 6 elsewhere appears to be used as 5.
Now, I strongly suspect that this was somehow intentional on my end, and that there's supposed to be a clue somewhere about how the scholars investigating that place/symbolic system have an incomplete and partially incorrect "Rosetta stone" (or else one that uses a slightly altered numerical system that changed over thousands of years) and the player is likely supposed to figure out the scholar's mistake through context by realizing that the letters containing that numeral are off ... BUT, that was three years ago (or four, I think, I don't even know anymore, feels like a lifetime ago), so assuming that this was really just me trying extra hard to be "clever," I once again disagree with Past-Me's decision very much, because I now feel like the puzzle is already complicated enough without introducing additional red herrings with transposed symbols and such. I think at the time I actually was afraid it wasn't difficult enough, which, now that I am having trouble digging through it myself I realize was not the case. I may have overcompensated a bit ...
Or maybe I just used my own early variant, which changed over time, but I totally forgot about making the adjustments on the poster? Or something.
But yeah, in short, you're not doing it wrong (with the exception of the numeral 2). On that one particular poster, the 5 and 6 are, indeed, transposed. So that is, in fact, 1451 and not 1461, as it may initially appear.
God, I need to go back to this game and maybe do, like, a "director's cut" where I cut some of the more BS steps from the puzzles and maybe spruce up the visuals a bit.