That's a Frigidaire Compressor. They were used on GM's, particularly in the mid '70's. you will need the brackets from a mid '70's GM car or truck with a sbc v-8 and a/c. Get all the brackets and pulleys. Good luck. Nickmil.
Yup, You are missing some brackets. It will work great for on board air. It has its own oil sump and an internal oil pump. You will periodically need to check the oil as you will lose some but not much. The biggest down fall is it is heavy. Nickmil.
That's an A6, you scored a good compressor. It's the only GM "log" type with a oil sump. It has a very high air volume(comparable to a York) and psi(300 lbs continuous, over 400 peak). I've used one of these for over 4years for OBA and it works great. Fills a 38.5x 14.5 from 10 to 30lbs in about 45 seconds without a tank.
Your missing brackets, one from the intake manifold to the bracket that you have. One that bolt to the front of the block and one that bolt to the front of the compressor around the pulley. If you still have the donor motor, get all the pulleys off it. It will most likely have a smaller waterpump pulley which is good for trail RPMs, the fan speed and water flow will be increased.