It's possible that Nortons were used in the 276 testing and that was why the half size ended up as the unmarked standard. The 16H was at the end of the 1930s the most produced model in Britain. Norton were making little else and, like Hawker with the Hurricane, may have been turning out as many units as they could afford prior to the Ministry actually ordering them. Both Amal and Norton were based in Birmingham.

Pursuant to the above Wiki states that Monoblocs use of idle jets meant they just had one standard jet block, and that that was a significant change from pre monoblocs, simplifying production. So it's very possible that a new [or even just later] carb set up for a 106 would not work as well with a 1065. My carb is however a war era one.