This is a seat rail that was used in an S15 Silvia. I was using it with the Recaro semi-bucket seat shown in the image. It's a bottom-mount type, for the driver's seat. It's an aftermarket product, and I've forgotten the manufacturer. Since it's common for the Silvia, it can also be used in the 13 and 14 models. I think it can be fitted to Skyline models if you modify the hole position on the rear side of one side.