This model AMAC came out in 1920. The main feature is the pressed metal cover that encircles the mixing chamber. This cover comes in two versions: yours is the type that suits a V twin, as it has gauze-covered air inlets at the sides, but is blank in the straight-ahead position where the fuel bowl sits if the carb is tucked in between the cylinders on a V twin.
Flange mount is a bit unusual, but they came in all configurations. In 1920 AMAC and Brown and Barlow (B&B) dominated the carburettor market, with Binks some way back.
Leon