According to Wikipedia the PU-16 was only found in the SCPH-5903. This is a very unique board in that it was only released for the Asian market, and was only in a single model, and it was colored white instead of grey.
The SCPH-5903 is the only PS1 model that can play video CD movies. The system also has the RCA video plugs like the original SCPH-100x models did.
Apparently there were only around 10,000 of these made, and they were primarily in Hong Kong. So they’re fairly rare. Because of this I can’t find any PsNee installation diagrams for the system.
Based on what I can find online the board most resembles the SCPH-1000. If you have one of these I’d look at PsNee diagrams for the SCPH-1000 and other earlier boards and try to figure out where things go through trial and error.
Here’s a video about the console: