An interesting niche, and an interesting bit of culture that's evolved around this product.
Sony's SRF-39FP Radio: The iPod of Prison (The New Yorker)
The SRF-39FP is the gold standard among prison radios in part because it runs on a single AA battery, and offers forty hours of listening time—longer than an iPod Classic. Digital models can require twice as many batteries, like the Sony SRF-M35FP, which runs on two AAAs. Federal inmates are particularly attuned to battery life because they are allowed to spend just three hundred and twenty dollars each month on commissary goods; more cash spent on batteries means less for snacks, stationery, clothing, and toiletries.
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...Radios like the one that was loaned to Demmitt are usually left behind by inmates who have reëntered the free world. Some prisoners believe that it is bad luck for radios to leave prison with their owners, while others believe that taking them simply violates the “convict code”...