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.
I sell MM3 modchips in my store.
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 MM3 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 MM3 diagrams for the SCPH-1000 and other earlier boards and try to figure out where things go through trial and error. In other words take a look at the PU-7 and PU-8 diagrams.
If you have a SCPH-5903 and are interested in having a modchip installed into it I'd be willing to do a free installation so that I can take good pictures of the installation and board (return shipping would be free within the US, but you'd have to pay for return shipping if you're outside of the US).
If you have a SCPH-5903 and are interested in selling it please contact me. I'd be interested in purchasing it if the price is right.
Here's a video about the console: