I just performed the region mod on my US Turbo Duo yesterday w/ an 8PDT switch.
After wiring everything & checking continuity, I sometimes had trouble getting a continuity beep out of my DMM when touching pins that I knew were shorted--I sometimes had to travel along the card slot pin until I got the beep.
Perhaps a thin layer of oxidation was indeed there, insulating the pins from my DMM probes.
Thanks,
-Jason