I found this reference at Oregon breakers:
Challenger became part of Westinghouse in 1987 and eventually part of Eaton, when Eaton purchased Westinghouse in 1993. The Challenger brand was dropped in 2005, however many Eaton labeled products can be use in Challenger panels.
You have westinghouse and challenger breakers in this box and a 1961 date. I would say it is a Westinghouse. I would like to recommend that you change the entire thing out to Square D load center with more spaces in it. I worked for two electricians in the past and they both put Square D in their homes, as have I. I just recently upgraded one of my load centers in my shop.
If not go get an Eaton part from Lowes.