Trying to determine what alternator this is. Believe it’s going bad as brand new battery is drained, and after jump start and running for a bit battery volts are reading even lower.

The cars an 87’ but I know the previous owner switched it over to a Serp Belt system as I needed a 88’ water pump when I replaced that.

 

That's an 88-92 style CS alternator. You should be able to buy a replacement for a V8 in that year range and have it bolt in.

 

Have it rebuilt..

 

What’s your reason for rebuilding as opposed to to just buying a new one? Just curious, I’m sure it’s not that cost effective, unless you DIY and that would be a first for me.

 

It is less expensive to have a an original alternator or even an orig carb rebuilt.
I have had the orig QJ & alternator on my '84 Trans AM rebuilt.
Have the chance to NOT rebuild using Chinese parts.
Who knows were the new alternator is built? China, Asia

 

Last time I took an alt in, it was 20$
Last starter was free. I gave the guy 20 for lunch. Just a ground

 

Looks like a 105 Amp CS130 made by ACDelco


$133.17 at Rock Auto.
You can buy a rebuild kit from Tuff Stuff for about $55 if you are good with your hands.
The new battery draining might be a bad alternator but it could also be a light left on or a bad courtesy light switch somewhere.
Try this
Charge the new battery.
Test the voltage. If you have a volt meter in the car with the key on but no engine running, note the voltage. It should be about 12.6 to 12.8 volts.
Start the car.
Check the voltage again. If the alternator is charging the voltage with the engine running should be about 13.6 to 14.7 Volts.
If the voltage stayed in the 12.6 range you have a bad alternator, if the voltage goes up over 13.6 volts you have some other electrical issue.

 

I’m gonna have it rebuilt found a local guy that gave me a half decent price rather than trying to tackle it myself. The battery is showing 12.5 when off, but I had it running after a jump start and it when down into the 8-9 range.