Technical details, specifications
Key to this models compactness is the use of a slim ni-cad battery in both the player and the headphones wireless receiver.
In 1988, Sony unveiled a remarkably small unit, the WM-505 (I can't confirm this was ever released in the USA - mine has Japanese writing on it...). There was no shape-shifting transformation needed to play cassettes, and the unit boasted wireless headphones. That's right, 20 years before Bluetooth you could jam out to your mix tapes without getting tangled up in cords. Sadly, I can't report how well it works as I don't have the wireless earbuds, which also would have been relatively new in 1988.
Key to this model's compactness is the use of a slim ni-cad battery (sometimes called a "gumstick") in both the player and the headphones' wireless receiver. Fortunately an add-on unit lets you plug in a regular AA battery. Also the playback head is tucked into the door like on the WM-10 to save space. All metal construction is thinner and more durable than plastic would have been, and does a superior job of protecting the expensive unit from inevitable knocks.