The 504A uses two silver oxide 357 button batteries. The black top is just press fit in. Gently pry it off. The batteries install with the positive side down. There are a number of batteries that cross reference to the 357. The silver oxide are a little more expensive, but last longer for this application.
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That tester uses 2 (two) LR44 (1.5 volt) batteries.
http://www.aikencolon.com/assets/images/pdfs/gardnerbender/Testers.pdf
See p.143.
The 504A uses two silver oxide 357 button batteries. The black top is just press fit in. Gently pry it off. The batteries install with the positive side down. There are a number of batteries that cross reference to the 357. The silver oxide are a little more expensive, but last longer for this application.