Yeah I never really saw the purpose of the 3 bay 4020s as you it can only accept 2 extra I/O boards and 4 mapnet cards, all of which will fit in a 2 bay cabinet. I have seen a newer 4020 with the 3rd bay used to house smart sync modules but on the older ones before smart sync, there’s really nothing you can put down there aside from modules. And yea typically a system of that size would generally just have a 4100 installed instead so there’s room for expansion as a 4020 of that size is pretty much maxed out.
When I saw that 2001 pop up on eBay, I recognized it right away. Those AC bell modules in the 2nd bay are very unique to it. I remember seeing photos of it a couple years ago from its previous owner who bought it from that school and was wondering what happened to it because I know it’s changed hands a few times since. Looks like a reseller got a ahold of all of his old fire alarm stuff. But it’s in good hands now, and it appears that it was originally uninstalled very carefully as everything looks to be in place which is good because those harnesses are next to impossible to figure out.