If the system was bought in 1995, it's probably a socket 5 machine. Personally I recall seeing IBM Aptiva machines in this form factor only in socket 5, nothing in socket 7, witch means the pentium 200 will not work on it, since that is a dual-voltage chip. What you need is a non MMX pentium 200 ( …

I gave up after doing more extensive research. It would seem the AGESA microcode in AMIBIOS is somehow embedded into a core module flagged as 1B (single arch bios) - sometimes even 1E, 2E and 3E modules contain microcode fragments. Apparently one has to extract said modules and add specific AGESA …