Not sure, but the bottom row is a 180 flip of the top row.
the squares are the most frequently occurring followed by rectangles. the other shapes each only occur once
I'm hijacking the top comment to restate what I think may be the solution and why.
It matches the Mirror/inversion theme. In fact it's a perfect match.
It reveals the 'up arrow', the only unknown character.
The 'up arrow' is the only character that matters, it points to the number 139. In other words the sign is just the address number '139'.
The 'up arrow' is a nod to social media where the puzzle will likely be posted/pondered and also the reward for the person that solves it.
That's my theory anyway. What say you?
Edit: Nevermind has solved it... https://pic8.co/sh/jkDJdS.jpg
(post is archived)