I bought this thinking it was for s13 sr20det, but from searching the part number it looks like it's from S14. It also looks a little different from the two on my cars. Is it from S14 and will it work on S13 sr?

It's an S14 alt, but I don't know if they're interchangeable... My guess would be yes.

I don't see why it wouldn't Nissan didn't change the alt going into the S14 AFAIK. I know it's very easy to use a KA alt on an SR, only the mounting tabs are different. Hell, I've seen people use Quest alts, which are a different amperage output.

Well from research I've seen posts where people say S14 alt can work with minor mod, and others say the later year 96+ s13 redtop/blacktop came with s14 alt. I don't want to deal with any wiring modification, I'm trying to fix that kind of stuff on this car. Already pulled out like 5 pounds of wiring crap done to the car. This alt looks a little different than the ones in the cars so I'm gonna pick up a for sure s13 sr alt just in case this one doesn't fit.

If you don't want to try installing the S14 one, go to your local parts store, and pick up one from a KA. You just have to drill out the bolt hole and swap the pulley....

Already have s13 sr one on the way.

The s13 sr alt I got was bad so I ended up using this one. It is slightly different from s13 one but it is straight bolt on, you just have to file out the inside of the + connecter. Other than that works great.