Just recently bought this s13, and Im told my rear brakes are definitely not the factory spec brakes. Any help is appreciated! Its 5 lug with drum E brake

https://flic.kr/p/2cWnUqp

Looks like a J30 caliper. The rear assemblies on most RWD Nissans of the time are pretty interchangeable, so I would suggest doing some Google image searching for vehicles like the J30 and Q45 and see what looks closest. If I remember correctly, the only RWD Nissan of the time that didn't use the drum style e-brake was the S-chassis. The Q, J, Z, and Skyline all used drum shoes inside the rotor. But my memory is a bit fuzzy on that last part.