Ian,
Yes, the diffuser throttle valve is a near solid lump with a small slide keyed into it for the choke.
The Sport versions of the 350 and 600 EW were fitted with twist grip throttle. So the F28 was advertised with a twist grip throttle and the E28 made do with the lever and the less sporty foot boards. Likewise the H28 600 OHV had the twist grip and the G28 retained the lever control. In neither advertisement is there any mention that a different carburetor is fitted based on the handlebar control.
The E/F28 handbook only mentions and describes adjustment of the plain jet B&B, with the variable jet superseding it in 1929 (though available from B&B before then.) In the 350EW handbook it states the air valve (choke) was only for starting; once the engine is warm the air valve is fully opened and the engine speed controlled solely with the throttle valve.
-Doug