Are you able to open up the keyboard? I would be interested to see if there's any additional shielding inside it. These things were designed not to minimize the amount of electromagnetic radiation emitted, as it is possible (given some fancy equipment) to use these emissions to see the contents of someone's screen, or what they are typing on the keyboard. These sort of systems were mandatory for certain government tasks, and it may still well be the case.
This is quite an interesting one, the EMR II has a D-SUB connector which I assume is electrically compatible with the DSUB connector. This one seems to have a standard cable with thicker shielding and some grounding to try and get rid of any emissions from the wire.