Chris,
Around about 1927 the 350EW brake cable anchorage moved from the rear of the fork girder to the front, though it did not show up in the catalog illustrations until 1928. So 1927 and on EW, the B29 and other 'sump model' 350cc, the A31/A32 and so on. It looks like in 1934 the lightweights had a different headlamp mounting, with four tabs on the leading edge of the girders rather than the old fashion 'headlamp irons' to the base of the steering stem. I cannot think of anything else that would narrow it down; these forks and top lug (with handlebar to the rear of the stem) were used for quite a number of years.
-Doug