I'm sure that a lot of people had to "nut it out". It's not instantly obvious and a buzz when you get it.
I didn't post detailed instructions but a little thinking and it all falls into place.
I love elegant engineering - simple solutions. Little things like the angled cut out for the plug that allows for different thickness doors and the fact that it will work on both left or right closing doors.
I purchased this about 35 years ago (yes I'm an old fart) and haven't seen one since.
Then again I haven't been looking. I'll keep my eye out. Might find some old stock at the local Locksmiths.
MrAnybody's request about seeing any for sale prompted me to do some searching. There is no patent number on it but I did find a similar device. It is more complicated, less strong, ugly and in general - less elegant.http://www.corporatetravelsafety.com/ca
There are also some VERY complicated (and expensive) devices to do the same job.
Search on “personal safety lock” and drill down a bit and you will find some examples – not the Master stuff.
If you haven't got one you can always use a chair wedged under the door knob