A few M/12s were assembled from extra parts on hand after the M/121 was introduced. My latest was made in Oct 1936. Its serial # is higher than yours.
The rifles were assembled after the receivers were dated. Sometimes they were picked over for quite a while resulting in lower serial #d rifles made (actually assembled) after ones with higher #s. The disparity you have is larger than usual. Your receiver must have been used right after it was made, then others were picked over for a while.
New info has surfaced since the book was published. Serial #835,17X was assembled in June 1934.
Model 12 serial #'s
Re: Model 12 serial #'s
If we were both polite and friendly you must have caught us on an unusual day 