OEM bios are different than retail for same board and the numbering usually will be different..the thing to compare is the release dates. The Sony 1004 could be the same as the Asus 1008...
WIll look around and see what pops up.

added: found Sony PCV-RX550 bios 1005 and compared to Asus 1005 and 1006 and they do not match even slightly....
The internal date string of Sony 1005 is 06/20/2002-I845 -P4B-LX
The internal date strings for Asus P4B-LX is Asus 1006: Award.06/18/2002-I845 -P4B-LX. 1005: Award.01/24/2002-I845 -P4B-LX
so as I said version numbers are different AND you cannot flash a OEM board with retail BIOS without an external programmer and it still may not work...

Here is Sony BIOS 1005 for your board: https://rebyte.me/en/sony/5553/?fileTypeId=4
It will not flash update IF it does not see the proper board....

Wow Thank you. It updated. It didnt solve the issue I was having (the cpu fan runs at full speed in windows 98 but not windows xp) but very cool none the less. Thank you for the link.

Good that it flashed. That was that latest BIOS I could find but will dig a bit deeper when I have time, maybe there is a beta like Asus 1008 (it is a beta bios) that Sony did but kept it hidden somewhere...

Hey there, I'm currently resurrecting my Win98 PC to get it up and running with WinXP. Back in 2019 I did a ton of work overhauling a Sony PCV-RX series. Man, the internet really is decaying fast, I can neither find the original forum that outlined the ability to install the 1008 BIOS on the Sony motherboards nor the actual files themselves. Lucky for anyone reading this, I've got everything you need in a pack right here left over from my old project.

Just came across this thread as I ran the genuine original recovery media on my PCV-RX550, and it came up with an error saying Windows could not be activated. My suspicion to why the fan control and other stuff on the driver disc did not work when I did a vanilla install of Windows XP was correct - it is because I had updated the BIOS from Sony version 1005 to a generic version 1007, which is just different enough to cause those incompatibilities. Had also noticed that the 1007 BIOS did not have a Sony splash screen and added the Energy Star logo to POST messages.

Hopefully the 1008 BIOS is an actual Sony BIOS and not the same thing as the 1007 generic BIOS. Worst case scenario I'll have to track down the 1005 Sony BIOS, as while I did back it up when AFLASH asked me if I wanted to... I re-formatted that same floppy disk like an idiot not long after without actually backing up the BIOS backup. Also hoping the 1005 BIOS does not cause problems with the 2.6 Northwood I have in here, because it'd really suck if i had to pull out the motherboard again just to downgrade back to the 1.5GHz Willamette. These PCV-RX series Vaios are cramped and not fun to work in.

response to my previous post: the 1008 BIOS is just the beta one from the Asus website, i.e. not the Sony BIOS I'm looking for. Go figure THE ONE website that maybe seemed to have hosted the Sony 1005 BIOS (seems to be called AWA005XDGI.EXE) only has a broken Cloudflare link. Was going to pawn off this machine with the factory OS installed, but I guess that's worthless now so Windows 98 it is.