In the 1950s and '60s Blaupunkt and other European auto radio manufacturers offered portable "picnic" radios that had the option of being installed in a car as a dash-mounted pullout. Unlike more recent pullout systems that were intended for theft prevention, the idea behind these older pullouts was versatility.
The Blaupunkt Derby 660 was introduced in 1965 so it's a period-appropriate accessory for a VW 1500. It's larger and more modern-looking than the previous Derbys and offers shortwave, longwave, and FM bands. I bought this one many years ago and found the under-dash mount more recently.
Nice typography on the dial still has echoes of the 1940s.
The car mount was meant to be installed under the dash. It carries the Ideal brand. Ideal was the original name of the company, and the blue dot that was used as a quality control symbol eventually evolved into the company's trademark. The name was formally changed to Blaupunkt in 1938, but apparently the Ideal brand was still used for some components.
A plastic ridge on the top of the radio case guides the radio and secures it as it slides into the mount. The following sequence shows how a protective flap on the mount automatically opens to accept the radio as it slides in. When it's pushed fully home the power, antenna, and speaker are automatically switched over to the car's components.
The radio is then locked in place. A push tab above the radio unlocks it and allows it to be removed again.
The radio also has jacks for an alternate external power source and external picnic speakers or headphones for use away from the car. They thought of just about every possible need.
In the last few months I was able to find instructions for mounting a Derby 660 in a VW 1500 or 1500S:
I originally bought this Derby with the intention of mounting it in our '65 Squareback, but I didn't locate the mount until after I sold the car. I'm not sure if it will find a home in my 1500 Ghia or not. We shall see.