The datecode on the chips that I can see are in the range from 8911 (!) to 9032...

There is what looks like a declaration ROM: a pretty much standard 28-pin ceramic DIP EPROM (12.5 V programming) with a 'PVAS3 4.00' label covering the window. There are three more labeled chips, 20-pin plastic skinny-DIP -- I think they're PLA's or something like that, but the inscriptions only append '.PV3' to the chip location (U8, U15, U26)

The idea of a non-standard ROM content that doesn't show up without the proper software, makes a lot of sense. I could read off the ROM contents on my programmer, but really don't know what to expect inside (or not) :-/

I gave the supplied control panel a glance thru ResEdit, and definitely has a dialog box with lots of options, plus some INIT code (nearly 20 KiB)

More info: I found a list of Sigma Designs monitors with no less than three PageView versions... the GS seems to have the same specs of Apple's Portrait Display, but the 'plain' PageView is the only one with confirmed 9-pin connector -- the other models say nothing about it.

The same web has info about the pinout variety of the DE-9 connector, but none of them mentions the PageView monitor...