I am cobbling together an 840av from parts found here and there (thank you LuckyBob for his case / power supply etc!) and managed to track down a motherboard on EBAY.

It was sold in "working condition" but there is a good amount of cap goo on the board and I noticed that it has a slot I hadn't seen before on the 840av motherboard - image attached - is this the infamous ROM slot from the early boards?