The best source, albeit in German, is the book "Eisernes aus Eisenach." It reprints many drawings and manuals, as well as timelines showing how the model changed, and at what serial #s.
Any bike starting with 3 is pre-war/wartime. After the war, the numbering started with 2.
Mine is a R35/1, engine 214599 and fram 224001. So engine is ~1949 and frame ~1951. I am working on building a pre-war one up from whatever parts I can find.... I already have a motor and frame, and just bought a pre-war final drive from Germany.
Many of the earliest parts were held over from the R4; their part numbers start with 204 (motorrad 4); these later became listed as 235 (R35).
Also see Dreher:
http://dreher-oldtimerteile.de/eshop/?l ... OmngVu6TEw.
I am in the process of creating translations from German into English of the parts lists. I also have a copy of the first Russian manual for the R35/1 (virtually the same as the pre-war bike) which appears to be excellent. They created their own manual, with new pictures, drawings, etc. Now all I need is someone fluent in both Russian and English, with a mechanical background! I am willing to pay for the translation work, so if you find anyone...