Ian,
Sort of a moot point now, but for the 1929 handbook Douglas started fitting the B&B with the needle to the 600EW. Still the two lever control, but semi-automatic; also called a two-jet model. The one with the gauze was called a plain jet model. Both plain and variable were offered side-by-side by B&B from 1924-28. Though not cataloged one could probably upgrade in 1927-28 by waving pound notes under the dealer's nose. Douglas' use of them in 1929 is an oddity as I suppose they should have been Amal, but perhaps Kingswood got a special close-out deal on the B&Bs!
-Doug