That's (almost without a doubt) either a Norton Model 18 (490cc) or possibly a Model 19 (588cc).
Probably 1929 (or maybe a little later) model?
The double barrel silencer/fishtail/primary case/engine/oil and fuel tanks and other parts are quite distinctively Norton.
1929 Model 18
Sorry about the strange front wheel effect but I didn't want to damage the spine of the book by flattening it any further than that as I scanned it..