I do a lot of work with power supplies, and as has been pointed out, the link (or a SPST switch) is pretty standard. In 240V mode, the link is O/C and the bridge does the usual thing. With the link in place, two diodes of the bridge cease to do anything, and the other 2 form a voltage doubler. In both modes, you get ~325V across the series combination of the smoothing caps.
Looking carefully at the manual - and without the actual thing in front of me - I can only assume that the Input Module has changed from that detailed in the manual (common with Philips/Fluke gear of that era). For that version, I bet that when in the 120V position (X4106), Neutral is going to the junction of the smoothing caps, whereas in the 240V position (X4102), it is going to one of the AC inputs of the bridge. Functionally, it's the same change that adding the link does in the example schematic in post #2 - just another way of implementing it.
I have *never* seen a PSU that requires both a link and an input change. But, there's a first time for everything; if you're sure that this one does, can I add my voice to those who wish to see a reverse-engineered schematic (won't take 5 minutes - just the stuff up to and including the big caps).
Is the spade post behind those caps labelled X4106? If so, perhaps it's a "legacy" thing? If my guesses are right, then the Neutral wire can be left on X4102 when the link is made (120V position). With the Neutral on X4106, the link status would be "Don't Care" - it is 120V operation, whatever the link says. Perhaps a bit risky! Maybe that spade post now serves some other primary-side function - AC fan, extended hold-up, DC operation? Who knows?