16bit is a hardware change, not just ROM.
This is what I've thought as well from my research. My card came with 16-bit working via its original 341-0266 ROM and even after PRAM reset, all I could get with 832 x 624 sense pins was a black screen when 341-0812 was inserted into the same card. Not even mono or 8-bit.
I'm curious to know the state of
@mg.man's card- is 16-bit selectable in 640 x 480, and is 832 x 624 a working resolution when using a monitor/adapter with the correct sense pins? Then it'd be good to see what changes with 341-0266. It may enable 16-bit color if previously missing, and it may or may not enable 832 x 624 depending on whether that was missing and the card is a Rev A or B or that information was even valid in the first place.
Have you tried it with the first gen ATi Driver? Doesn't that apparently, completely weirdly, likely due to being based on Apple 8•24 GC example code, enable acceleration on the GC?
I did try to make this work a few times, but it seemed to always show an X when loading, and acceleration wasn't seemingly any different. The 7.0.1 CP works with the same caveats all the way from when it was seemingly broken by Macintosh Drag and Drop v1.1 around 7.1.1 > 7.1.2/P all the way through 8.1 on the 030 machines. I believe the incompatibility on Quadras was due to the busmastering pins being configured before Nubus 90 required the same pins for different functions (which all Nubus Quadras at least partially implement.) I haven't tried the ATI drivers there but I can attempt. Maybe x100 PowerMacs too?
Hmm... in my limited experience, 'higher' part numbers usually = 'newer' things... You're saying my -0812 is 'older' than your -0266? Is yours an EPROM or Mask ROM? Mine appears to be the latter, though it could be a EEPROM I suppose (but back then?).
I was under the same impression; and it confused me when I was missing Rev B features... Speculating based on the lack of 16-bit/832x624 support when installed and the ROM sticker itself is copyright 1989-1991 on my 341-0266, but curious to know what yours does out of the box.
The chip is an AM27C512 -200PC (the UV erasable variety.)
If so... does that mean my GC can't do 16bit color? = the 0266 ROM will be no good to me?
The serial number of my card does start with CA2... Still, give it a go.