According to the sales brochure picture of the 1953 Bullet, which you can find here in the Photo Gallery - Original Sales Brochure pictures:
http://www.hitchcocksmotorcycles.com/ph ... category=5 it has a square air filter fitted underneath the seat on the off side. This looks just like the same filter as on my 1953 Model G (except for mine being removed to make space for a Concentric). You can find the part number for this filter in the Parts Books Online - 350 cc Rigid frame - 1953 Model G section:
http://www.hitchcocksmotorcycles.com/pa ... Rear_Stand The filter has a felt insert which I guess can be washed a couple of times before it myst be renewed. Alternatively use a conical filter like I do, or a bellmouth with mesh if you're not riding on dusty roads. I do that too, and indeed it makes a difference due to the improved aerodynamic shape of the intake. Regards, Anders F. R.