I’ve only just come across this. I believe they should all be 40.5. I have a POH here that lists total useable as 81 USG. I think the 46 per side comes from a later model or perhaps even the turbo.
I’m guessing this one is based on the 40.5 per side variant and as such it’s only the gauge that is incorrect.
If looks like whoever did this got lazy or just plain forgot to change the gauge later because that is the fuel gauge from the C172. The needle doesn’t even move across the whole scale. It certainly needs fixing.