"Bad" Design is on purpose. Your task is to fill the two empty spaces with the correct sign. Edit: I'm sorry if it's too hard
This is by no means an answer, but here is one thing that I've noticed:
There appears to be a series of "L"s forming the shape of an "L", which makes me think that empty space needs an "L":
That will also explain the "bad" design
because if the "bad" design weren't there, it would be to easy to make out that pattern.