This is a "2 in 1" Motometer gauge : fuel level (with a reserve light) and oil temperature...
I bought it totaly NOS, in 2000, on eBay : complete in its original box, never mounted! For a 50 years old accessory, this is pretty rare!
So, some paint and 2 holes in my tank later, I finally have a nice jauge on my dashboard!
Just one more accessory : a Smith tachometer.
It was not that easy to understand the way it works : by induction!
Anyway, now it works perfectly... I got all the tools to keep an eye on my little engine!