The book Classic Motorcycle Engines has a chapter on the 1955 works BMW flat twin. I've uploaded it to:
https://drive.google.com/open?id=1gLaaD_xzHFQNXzSrnkPifBdrWnM_IWkgThe book was published 1986 and had detailed technical descriptions with historical context on 20 outstanding engines. In the case of the BMW chapter the engine under discussion dates from 1954, so the drawing and original discussion 1955, and the historical discussion dates 1980s.
The technical description of the engines is incredibly dense, packing an almost unbelievable amount of information into each paragraph. I dipped into it tonight and I'm sure there was information that wasn't there yesterday. However, if you don't read it attentively you might miss things like the reference to the 500cc flat twin with desmodromic valve gear, unfortunately abandoned because of lack of funds.
Full citation is "Classic Motor Cycle Engines: a new perspective on 20 outstanding designs", Vic Willoughby, 1986, Motor Racing Publications Ltd, Great Britain.
Enjoy!
