Correct! Built for Max Hoffmann. I can't find any definitive answer for this, but I presume that the two-cylinder engines joined together to make a 'four' were Fiat 500...
Fiat 500 had a 2 cylinder in line engine. But the Austrian Steyr-Puch 500/650/700 version had its own two-cylinder boxer by Puch.
Apparently, the boxer engine intended for this car could have been any one of three: Type 750 Boxer 4 (985cc), Type 752 Boxer 4 (1286cc) or Type 706 Boxer 2 (749cc). I don't believe that a four-cylinder boxer engine was ever fitted in the end; equally I don't know what engine the (surviving) car has fitted.