I guess I don't understand the question. Since Asus's site doesn't make a distinction for rev 1.04, I'd use the BIOS they provide for plain P5A-B. And the manual says use AFLASH.EXE. Which is all perfectly standard, so it can't be what you're actually asking...

So if you don't mind me asking, I'm intrigued - what are you worried about that I'm being blind to?

First off, thanks for the reply! Nothing out of the ordinary really. What you consider "perfectly standard", as you stated, wasn't really all that clear! I shall use AFLASH.exe then, thanks for clearing that up!

As far as the revision goes, the P5A and P5A-B came in multiple revisions, and the later ones (1.05 and 1.06) will not work properly with the K6+ CPUs (they run horribly slow). It's probably just like you said, that there's no distinction between the revisions and BIOS updates should work just fine across the board.
However, I'd prefer if someone that's tried the latest BIOS would post his opinion on it. Is it totally safe?

Can't really speak for P5A-B, but I use Jan Steunebrink's 1011 Beta 5 BIOS on a rev. 1.04 and on a rev 1.06 P5A board. I bet it's totally safe. 😉

Yeah, that about settles it, since the boards are largely identical other than the form factor.
Thanks for helping out, I'll update the motherboard ASAP 😀.