Michiel said:
There are actually quite some more boomboxes without internal transformer...
... and some that have either/or power supplies. Technics SA-C05L for one, it can take 10 Ds, but only when you remove the mains TX, a bit pointless since the tape motors run down thae 10 in under 30 minutes to the point where they won't.
Hacker SP80's battery tray was an optional extra and again, the mains TX had to be removed to use it. Some Roberts sets had the option of six Cs or six Ds, if you fit the battery that holds the six Ds, there's no mains TX but on the battery that uses the Cs, there's room for the mains supply on there.
But on rare and small, is the Japanese JVC RC-N5 not quite a rarity? Few enough were built in any case but most were out of Singapore, this one is Japanese, uses six AA cells to allow space for its onboard mains and has a hidden cassette deck: