Got an old, old square D panel. No markings on breakers and panel is an NA1B. breakers are about 6" long from center of the panel to the outside edge of the breaker and nothing available at two local suppliers. One is really good at getting me oddball stuff.
My only hope is one of those extention lug kits that lets you use a smaller breaker on a panel made for the larger breakers. I've seen them but don't know what they're called or who makes them. Know what I'm talking about? It bolts to the new breaker on one end and the panel lug on the other.
I could replace the panel but that would be a job since the panel is piped to two other panels side by side and all are set in a poured concrete wall.