The Challenger product line was purchased by Eaton/Cutler Hammer. Their web site has a search function that will allow you to enter "Challenger" and locate documents that list acceptable (as in legal), if any, breakers for use in the panel, as well as letters from the CH engineering staff informing electrical inspectors of the acceptable use of Eaton/Cutler Hammer breakers in these panels.
Many of the panels available today will accept breakers made by other manufacturers, but only listed or classified breakers are UL certified to work in other brands of panels unless you have an exixting manufacturer supporting one of the orphan brands through documentation.
While "will fit" breakers will undoubably work short term, there may be interruption rating issues and compatability issues with the buss material. In the event of a damage (fire) claim related to electrical service equipment failure, breakers OK'd by the manufacturer to be in the panel is a point in your favor.
Challenger breakers hard to find
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