I did some Googling and found out all I could about it. It was apparently the second most popular construction toy after Lego, but tiny in comparison. It's big in the US and Holland, not so much here.
As the owner of more than 30 old slot machines I decided that the first thing I'd make was a fruit machine! This enabled me to learn all about the techniques needed, but there were plenty of things which were clumsily done.
Here's a video of that first machine (the quality is awful, because I was using what was then a 10-year-old digital camera which didn't have a proper video function). It was loosely based on a Beromat mechanism:
I subsequently made an amusement machine:
and a coin pusher:
and then another amusement machine:
and then a smaller fruit machine:
and now I've just finished an even smaller fruit machine, using all the knowledge that I had acquired over the years.
As you can see, the word 'Slot' in the heading is in quotes because the machines actually work with balls!
Who knows─this might inspire some of you to do something similar!