Hm...
Thats a good question walkgirl.
The Stereobelt was never produced for public sale to my knowledge. There were only a few units actually built. Sony did finally agree only a few years ago ( after Morita's death) that Stereobelt patent was the first portable cassette player patent filed. But with no actual factory production or sales of any of these to the general public the Sony TPS-L2 still stands IMO as the first portable stereo cassette walkman.
Pavel did invent the first portable cassette player of note I believe he applied for patents not only in Germany but also Italy and few other countries about same time so Germany would not be the actual home. I believe he was living in a South American country when he made the first one. Check out this topic for more details.
Stereobelt topic
The first 'Walkman' was the TPS-L2, that is a 100% certainty. The first portable stereo cassette player which used headphones was far less clear. It could be argued that the 1961 Phillips stereo portable was the first, there is no clear definition.