I bought a house built in 1963 with these breakers.
Didn't know anything about them until I upgraded to an electric tankless water heater.
After reading the guide by gcpee, I thought I would share my experience.
My box has the screwdriver adjustment that 'locks in' the breakers. I am not an electrician, but I can see that this is poorly designed.
I had to buy several different breakers due to to the small size of my box and the fact that I had to add second double pole breaker circuit.
I was told by a friend that was helping me that I should get an electrician to change out the box to a new, larger unit, but I knew that would cost hundreds of dollars and since this was a relatively small project, I did not. The project is now complete, but the setup has several issues that still concern me. One of the contacts on the bus bars is discolored and appears to have had some arcing in the past. One of the two 40A breakers trips when used in this position while the other does not. I have to use that slot because there is no more room in the box. Also, some breakers will pop out with the slightest touch and will not line up with the others. The box itself is very crowded and there is little room to run the wiring around the breakers,
Three other problems with these breakers are the fact that even the newest ones are over a quarter-century old, the high price and availability..
Electric equipment does not get better with age and these can be very old. They do not have the date of manufacture on them, so there is no way to tell how old they are. They were made from the 1950s to 1980. They can also be hard to find and expensive, depending on which one you need. While I love eBay, it would have been much easier if I could have gone to the local hardware store and bought new ones in the sizes I needed. As far as cost, I had to buy some breakers for my sister's house and was able to buy a 100A double pole for $30 and some 20A single poles for around $7-10 bucks at Home Depot. I have to admit I was jealous at the prices and convenience.
The info at the link in gcpee's guide is very good. I also found a pdf document that was very informative at
http://www.codecheck.com/pdf/electrical/240overcurrent/FPE%20Article%20from%20DH%20-%20Nov2003.pdf
As it is now, even though my project is finished, my current setup has me concerned about the danger of a fire and I plan to have the entire box replaced later this year with a Square D unit and all new breakers when I get the money.