Originally Posted by
DannoXYZ
Some locknuts have a wrap-over cap that doesn't allow the steerer to poke through. These locknuts stop threading down when cap contacts top of steerer. I know for sure Campy and Tange headsets are like this. Not familiar with that vintage Ritchey though. As others mentioned, a washer or two between locknut and adjustable cup will allow locknut to press on cup.
I've never seen a locknut that positively stops at the steerer top but I've seen several with a plastic O-ring that makes them a tight fit on the steerer if you try to thread them down too far as does act as a sort of depth stop.