I've been going through the AC system and finally had a look inside the (early type) evaporator. Thought it would be good to share some pics, as it is probably a ripe area for maintenance, especially if your system is actually working and creating condensate!
Pic 1 shows the unit upsidedown, illustrating the sealing between the main unit and the condenstate tray (black plumbers putty everywhere!). Globs of cork insulation were used to seal the coil tube area (it was removed prior to this shot; I now recognize as cork tape). Hard to believe that the putty is 'normal', but you never know.
Pics 2&3 show tray and tray detail, evaporator core removed. The drain tubes are positioned relatively high in the tray so it seems like there could be condensate sitting in your tray for extended periods of time (maybe finding rust is not surprising).
Pic 4 shows what came out of the assembly. Two thin sheets of styrene insulation surround the core and they held up perfectly. The other foam in there didn't, I'm not sure where it was used as it was literally dust.
I've since blasted and epoxy primed the pan, amazingly it's going to be fine. Looking to seal things up further and possibly fit some some aluminum scrap to help close the major gaps that the cork tape originally filled. Hoping to clean up and use 'as-is' otherwise, will advise how this works out