This was the Sdkfz 251/17 Ausf D, with a compact 20mm mount. It sounds like it could serve as an excellent makeshift APC/MICV in the late 1940s, early 1950s...
If some of you WW2 buffs are interested, I stumbled across this yesterday.
There's a number of SdKfz-251 variants with unusual weaponry, most of which mustn't have been produced. (scroll down below the Star Wars blueprints, and that was not what I was looking for either!)