[cctalk] Keyboard Blockers?

[Douglas Taylor via cctalk]

I picked up a keyboard for a Wyse terminal at a flea market the other 
day.  When I tried some of the keys, they couldn't be depressed.

I thought, at first, some dirt or debris had gotten stuck there, but on 
closer look I saw something black below the keys that seemed to be 
stuck.  I pulled a key cap off and found a U shaped piece of black 
plastic that was put there on purpose to prevent you from depressing the 
key.

The question came to mind; "What sort of application would be so crude 
that you would have to prevent the user from depressing certain keys?"

One of the keys was a Break key, which sort of made sense to me, because 
it would halt a PDP-11 if that was the host machine.

This was the first time I had ever seen this kind of thing, was this 
common long ago?