That third dot inside the bottom of the 5 throws it off a bit. If your mind reads it as a divot then the shadow it casts makes the number look raised to match the shadow.
If your mind reads it as a raised bump then the shadow it casts makes the number look sunk.
Which is all the more confusing because the nailhead bumps cast opposite shadows of the divot.
Nice find!
This phenomenon is just like the moon crater illusion referenced by @Darren_Garrison upthread.
Except, somehow, different. The moon crater illusions have always been consistent for me, depending on the specific image.
I kept looking at this thing, and finally copying it into Photoshop and flipping it upside down and sideways, and all I could ever see was the “5” as a surface elevation.
I was going to post about how I was the only one apparently seeing it as a surface elevation, but then I got distracted and went on to other things. Late at night, I happened to look at the image again, and it absolutely looked the opposite – now it was distinctly recessed, and remained so no matter how I looked at it!
Weird indeed. Makes me wonder if one’s state of mind – for example, tiredness due to lack of sleep – can influence that perception, in the same way that auditory pareidolia can occur due to lack of sleep.
Take a look at the second example I posted, with the plastic plates. It is probably closer to the letter.
For me, that one is absolutely consistent, like the moon craters – it’s a projection or depression depending on how the image is flipped. The OP image is not like that – it’s either one or the other – I’ve distinctly seen it both ways and I have no idea what determines how I see it! 
Jerry Andrus was known for illusions like this. Jerry Andrus - Wikipedia
He designed a paper cutout you can make (not identical, but similar):
https://www.gathering4gardner.org/wp-content/uploads/2021/03/3D-dragon.pdf
Nitpick: that page says that the dragon cutout was designed by a team inspired by Andrus, not that it was designed by Andrus himself.
Andrus’ dragon illusion has been presented in different colors, with different credits and logos. On Phil Plait’s page (The Bad Astronomer) it says “Created by Jerry Andrus.” I think it has been adapted by many. To see more, on Google’s Image Search page https://images.google.com/ try using the terms “Andrus Dragon.”
Bravo.
Interesting. No matter how I look at it, I can’t see it as anything but stamped ( sunken) rather than embossed. I can usually flip these things in my mind at will.