What's the resistance? Perhaps one adapter has slightly higher resistance than the other, causing the keyboard signals to be out of spec for the KVM?

I've an odd problem with one of my Belkin KVM's where a PS2 keyboard works perfectly when connected to a external keyboard adapter on a ZX Spectrum, but via the KVM the same keyboard is completely dead.

In theory PS2 KVM's should pose less of an issue with connectivity than a complex protocol like USB, but it's still a bit of a lottery.

I'm wondering if one manufacturer broke things by trying to "bulletproof" it with tie ups or tie downs or suppression caps.

So the KVM PS/2 connector only works in one of the AT to PS/2 adapters? But all other combinations of keyboards and adapters work ok? Maybe there's some pin/socket dirt/corrosion problem that lines up just wrong with the problem combination? Did you also check if the unused pins and the shield are connected through for both connectors, in case the KVM needs the shield as a reference for something?

Thanks for all the suggestions. As I was poking around with my breadboard leads to check everything per your advice (resistance, connection of pins to the shield, etc.) I realized that two of the pins on the PS/2 side of the faulty adapter (the DAT and unconnected pin 2) were much less "tight" than the other pins (the other pins, and all the pins of the working adapter, have some resistance when you stick a breadboard lead in).

My guess is that the connector pins on the keyboard I've been using (and the breadboard leads I've been testing with) are either thick enough or long enough to make contact with the loose DAT pin on the faulty adapter, but not the pins of the PS/2 cable I'm mating the adapters to. (Indeed, when I connect the adapter to the PS/2 cable, I don't get a through-connection from the DAT pin on the DIN end of the adapter through to the PS/2 cable's DAT pin on the other end).

So this looks like it may just be a faulty PS/2 connector on the adapter rather than something intentional; I didn't find any pull-ups or pull-downs on any of the pins, resistance on the line more than 1 ohm or so, or any connections to the shield (besides the shields themselves). I appreciate all the suggestions since it did help me find what I think is the cause!