Too bad it is hard to find see the complete licenceplate number in that angle.
As you say it do have a lot of details that could indicate it to be one of the prototypes.
In the more known photos of the number one prototype you can see a similar centermount antenna.
Also if you look at the top of the door the number 2 prototype have the same bend upwards towards the A-pillar as the later production Hebs have whereas the doors on prototype one looks to be almost vertical all the way. Hard to see on the photos I have here at work. Also the semaphore location looks very correct.
One thing that gives me a slight dought is that the cabrio top doesn't fold down as low as on the prototype number 2. I'm not sure if I have seen any photos of number 1 with the top down.
Also the Radcliff roadster's top folds down lower than on this one.
Interesting photo nevertheless.