In my 1990, there was a single wire cut-out on the back of the original compressor that I had to change to 2 wire when converting to R-134, which I think had a similar function to the later systems. On my 92, no such switch is present (and there is no wiring for it either). Wiring manuals for both years match up with this reality. No switches are present at all on the condenser output on either. I think that the recirculate function is controlled by this switch in the 1990. When turning on the AC, it will blow for about 20-30 seconds, then you can hear the air start to recirculate. Since the recirculate option was added to the control head in 92, this was likely the reason for dropping the switch in that year (although the compressor is the same and just has a plug in place of it). When trying to source lines for the 1990 truck (NOT easy for a 454 since they have a unique mounting orientation and high side valve location), the only two that I could find were for 92 and 94, and the pictures did look like the 94 had an additional port like you mentioned.
Both trucks have the auxiliary cooling fan as well, and the wiring looks identical. Power is wired directly from the power block with a fusible link to the relay, ignition power is sent to the other contact, and ground is provided through the switch in the head. Simple! The 1994 AC controller and ecm seem pretty similar to earlier models, despite the switch to R-134, so it seems unlikely that the operation of the fan would be completely different (especially if it involved making special wiring accommodations just for 454 trucks). Now, 95 is an oddball in all directions, so I'm not so sure on that one, and I can't find a 95 wiring manual to study this. When inclusion of the cooling fan expanded beyond big blocks, along with improved computer functionality, it makes sense to make more efficient use of the auxiliary fan.
Thanks for the detailed summary, I’ll study it and follow up with questions.