It is running some AMI BIOS variant with a copyright date of 1992, I currently don't have the exact version string around to compare with the ROM dumps on retroweb. vbindiff says the "F" and "M" images are identical and the "H" only has a few 1-byte differences, mostly typos in ASCII strings.

I've written a small boot sector program once that tries out memory and CPU information gathering techniques, so I know the INT 15h, E820h, E801h are not implemented but INT 12h and INT 15h AH=88h return something sane. When I have more than 16M installed, the later reports the full 31M of HIMEM, but I'm not sure how the ISA memory hole factors into this.

From what I saw glancing at the scanning code yesterday, syslinux 6.x should fall back onto AH=88h if AX=E820/E801 doesn't work. It's interesting to know that this worked in older SYSLINUX, I'm curious to check out what changed.