There is a photo here of an elastic contact switch supposedly made by the Maxi-Switch Co. that seems to be a derivative of Alps KEH series:

Model 4 membrane keyboard - anyone know how to fix it ?

If anyone ever finds a reason to open up an early Model 4, I would like to know what part number is written on the PCB of that keyboard. It may suggest the series name for those keyboards and switches. (There are a few advertisements out there from Maxi-Switch, but not enough to get details on most of their series.)