Which breakers are compatible ?

[Smart $]

Funny, the load centers I am familiar with all have a label somewhere indicating what breaker types and sizes can be added.

But are they made by another major manufacturer or an acquired one, such as Westinghouse or Crouse Hinds in a Cutler hammer panelboard. I don't have access to any panelboards to check, but I do know there is no UL requirement to mark a panelboard for such, and I do not see why any manufacturer would unless it's an in-group brand as just mentioned.