@David Cook I did some L2 cache tests with my 7300 a while back that mostly match yours. I thought it was interesting to see how more L2 cache also improves disk and built-in graphics speed.
L2 Cache modules tested: None, 256KB, 512KB and 1MB
Spoiler, more L2 cache is generally better, but...
No, this wouldn’t be like how a BlueSCSI disk image works. Someone else might know a way but I think you’d need to install on the new disk (actually an SD card in the adapter) like how I described above if you used a real IDE spinning disk drive.
To use the IDE drive:
Plug it in where you removed the disk that came with it.
Download and burn a CD-ROM for a Mac OS installer (like the first link here foe 8.0: https://macintoshgarden.org/apps/macintosh-system-81-mac-os-81). This of course assume you have a computer with DVD/CD burner and...
That IDE drive would work (of course you'd need to format it which would erase the Windows 98 install). Is your BlueSCSI an internal 50 pin version? You could use that in place of the CD drive but that might make installing a new Mac OS harder ...unless you use a HDA image preloaded with an OS.
It looks like it, any vendor markings on the other side? What brand was the video card?
I have a Supermac Dual PDS adapter that looks similar, but I guess there is a chance it could be a combo Cache/accelerator slot and PDS slot adapter. I have one of those from Daystar that looks similar too.
That seems pretty definitive that is a prototype "Alchemy" board for a 6360 or 6400.
The fact its in a desktop case makes me think its more likely a prototype 6360, but I guess Apple could have been testing the faster 6400 series...
Wonder what the gestalt machine ID number is? Something like Tattletech should be able tell you.
Not sure if there is a better list, but found this:
If its a prototype 6360 or 6400 I think it would be 58: