They preceded wire nuts with the spring. We still see them but are not very common any more. A better connector where vibration was an issue but the caps have been seen to loosen too.
They do not bear that wire combination anymore on the new ones. but if you look at jdevlin link from above Marrette now has # 63 and # 65 connectors that are certified for that wire combination. oh here is a direct link
I remember seeing those setscrew connectors at the local hardware store when I was little. I only seen one once in the field, and that was in a motor's terminal box. Funny thing only one connection had the setscrew connector, the rest had wirenuts.
I know they list them in the current ideal catalog.
Even though I have been around electrical work for 30 plus years, and all the guys I talk to at the supply house, many have seen these types of wire nuts but few actually use them. The other thing is that you don't see them every day. You can go for months and and even a year or two then you see them again.
