From what I can see, it's no older than 1935, that was 1st year of the rectangular tool box and horn mount under light. It's a 74 or 80 cu in. Flathead, sprocket is on the Left side. It doesn't appear to have the horseshoe oil tank that came out in '37.
Above is based on my interpretation of photo against HD data book, not an expert opinion.
A guy I ride with rides that bike as a bar hopper it's a 39 flathead. I will see him soon to confirm. Believe it or not in October he is racing that bike and other bikes on the beach at wildwood NJ. YouTube race of the gentlemen wildwood and you will see them racing.