Hi Aero Nut,
The EH prefix engines were fitted to the 1927/28 Douglas model SB27 motorcycle. These engines are 600cc. Douglas also produced a version for use in industrial trucks. I have looked in books by Jeff Clew and Brian Thorby but cant find any reference to these units powering any aircraft - but this doesn't mean they weren't used - there are a couple of references to earlier 4HP (600cc) engines being fitted in aircraft.
Hope this is of some help,
Regards,
Eddie.