Hi Mel,
I believe your challenge is Franz Xaver Mehr's Me.3 ?? It looks to be fitted with a 20 hp Ursinus horizontally opposed twin engine. It was built and tested at the Friedrichshafen factory in 1931.
There seems to be some confusion as some sources refer to it as an Erla-3 and say it used a DKW air-cooled inline 2-cylinder, but I think this referred to a later rework of the Me.3 because Erla Maschinenwerk wasn't founded until 1933 and as a joint venture between DKW's Jørgen Skafte Rasmussen and Franz Xaver Mehr who had producedhis earlier designs as Mehr Flugzeugbau Meckenbeuren