Many thanks for everyone's help...

I thought I should reply just to close this out for future reference. I bought the Klann shoe; which happens to be a drop-in replacement. It fit the rail perfectly and I just need to wire it up... it looks very similar to the Allen shoe it replaces.

The folks at Klann were very happy to simply sell me the one shoe with the in-place potentiometer, etc...

I did order a Rogers shoe - someone was selling them from a lot of several he bought from the Rogers warehouse. This shoe _would_ have worked, but didn't look anything like the previous Allen shoes...

Anyway - if someone in the future is looking for a replacement for an Allen shoe with an existing potentiometer; just call Klann!