To understand the BIOS, we must first understand the console it came from. The SCPH-5500 is a Japanese model of the original PlayStation, representing a significant transitional phase in the console's lifecycle. Released in May 1997, it was part of Sony's third major revision of the hardware, following the original SCPH-1000 and the SCPH-3000/5000 series.
Upgraded to use a metal beam support instead of plastic, drastically reducing the FMV skipping and disc-read failures common in early units.
The correct scph5500.bin should have a size of exactly 524,288 bytes (512 KB).