I got this at a garage sale along with a bunch of other stuff. I have no idea what it is.
It says Pro and Made in Taiwan on one side and 40 on the other side.

Notice that the carb is backwards -- needle on the right and throttle on the left. Also, the throttle is backwards (push forward for idle, pull back for WOT). If I turn the carb around the fuel nipple will be facing the prop.

That is a Thunder Tiger pro .40 also if you turn the carb around all you have to do is loosen the high speed needle assembly and adjust the fuel nipple, you should be able to turn it.

It's a Thunder Tiger Pro .40. Turn the car around. Then you can loosen the fuel inlet nipple from the carb, rotate the inlet to the rear, and re-tighten.

For a long time, the Thunder Tiger pro .40 was the only approved engine for AMA 424 class Q500 pylon racing. They are well made, run well, and now apparently discontinued, unfortunately.

For racing, we used an APC sport 9/6 prop. Good engines would turn that prop nearly 17,000 RPM. Great ones a little over 17K. Rear bearings originally had a way of coming apart at that RPM, so mostly they were replaced with better bearings with plastic ball retainers, vs. the stamped steel cage of the OEM bearings. At more sedate RPM's (like with a 10/6, etc) the original bearings would probably be ok.

That engine is one of the earlier ones. Later ones used a needle valve assembly that angled back, keeping your fingers further from the prop. Depending on what you're going to do with it, you might want to pull the back cover off and see what sort of bearing is in it.

actually the carb is mounted upside down

As pointed out you have the earlier version of this engine which was made in Taiwan. Later they were made (or at least assembled) in China. Those of us who have raced the TT Pro .40 prefer the Taiwan built engines over the Chinese built ones. If your engine is in good shape, it might be desirable to some modelers. Also as was pointed out, for some reason your carburetor is not only in backwards, but also upside down. In other words the part of the carburetor that would normally go into the crankcase is pointed up and functioning as the venturi. Strange to say the least! If you want to restore it to normal design parameters it would need a new carburetor, or carburetor body.
Unfortunately parts for the whole Thunder Tiger line are hard to come by, since no one in the US is supporting the engine, and they are no longer being produced. To reinforce the last comment about the original bearings, they really would explode, and hand grenade the motor. The races failed, and broke into pieces. The good news is that these engines are very powerful, and very reliable.

The fuel inlet doesn't rotate on these, just put the carb with the regular side down and everything will be back to normal...

I agree someone put the carburetor in upside down into the engine spigot. Just turn the carb over and put it back in. Then you will be in business.

Jack, We keep trying to help with small details. I measured both ends of Thunder Tiger Pro .40 carburetor and there is a couple of thousandths difference. The end that belongs in the crankcase is the larger diameter, so it should be a little snugger when properly assembled. Also, note that it appears that the "O" ring is missing that goes between the carburetor and the crankcase. Any black auto parts store, or Ace Hardware store "O" ring should work. Good luck, Greg