A bit off topic but I have a question regarding and old ILS/VOR radio navigation system I'm working on.

It is composed of the ARN-12 Marker, ARN-14 VOR and the ARN-18 ILS receivers.

Studying the schematics I noticed that the control box of the ARN-14 C-996A has provisions for the frequency selection of a DME (Distance Measuring Equipment)

The ARN-14 is from the 1950's and I cannot find any DME reference for that time period