So I gave up and just bought 2-ISA P3B-F. Managed to get rev. 1.04. And I was lucky and got it for like $30 incl. shipping. Minor damage on IDE ports but overall pristine condition, though I don't know if it really works (don't have time to test right now).
Just documenting my latest findings on adding second ISA slot if someone will find a courage to actually do the thing.
The main reason I gave up was because of the difficulty of bending ISA slot pins in Harry Potter scar fashion. In my opinion it will be very hard to do it properly with just a pair of pliers and you'll need some DIY tooling to do it right.
I've searched the Internet for the source of the already-bent ISA slots and came to a conclusion that it was a custom order from ASUS (like their custom ASICs) and it's not possible to buy them separately. Though I managed to find two other ASUS mobos which theoretically can be used for cannibalization: MEW-L and P3W-E - these are again "variable ISA" variants which 2-ISA versions I actually saw on the Internet. And they are low-end Intel 810 boards with integrated GPU and no AGP - not such a big loss. The difficulty of desoldering such a big element (without damaging it!) from a multi-layer mobo which was designed to dissipate heat is a whole another topic.
Also, bear in mind, that desoldering 6th PCI slot and soldering 2nd ISA in it's place is not the end of the story - you should also get rid of the AS61256-8 IC with accompanying elements and solder two 0Ω resistors. AS61256-8 is a custom ASUS PCI arbiter that allows to extend the number of PCI slots - it is needed because 440BX own PCI arbiter supports maximum 5 PCI slots. Because you're lowering the number of PCI slots to 5, this additional PCI arbiter is not needed anymore. P3B-F schematics include relevant sheet that describes it (AS61256-8 is named HT82A001 on this sheet).
1 vs 2 ISA slots:
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PCI arbiter schematics:
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TL;DR:
I recommend just finding 2-ISA P3B-F. They are rare, yes, but this is possible. Though it took me like two years to find one for a decent price.
Another "low-effort" alternative is using ISA riser, as was already mentioned by zwrr:
zwrr wrote on 2024-01-10, 03:39:
What is your purpose in adding a second ISA slot?
If in order to use multiple ISA devices, why not try this.
Short on ISA slots? Try this.