PART NO.  SPL.PAGE  DESCRIPTION                                                           
AHH5601   15.R.42   Air control cable and Knob Assembly -(heater, 1500)
AHH5601   16.R.45   Control cable and knob assembly -air -(heater, 1600 and 1600-MK-II)
AHH5602   15.R.42   Demist control cable and knob assembly -(heater, 1500)
AHH5602   16.R.45   Control cable and knob assembly -demist -(heater, 1600 and 1600-MK-II)
AHH5786   TC.R.56   Control -push-pull -air (long) -(heater, heat+vent eqiuip, Twin
    Cam)
AHH5786   TC.R.57   Control -push-pull (air) -(fresh air vent equip, Twin Cam)
AHH5787   TC.R.56   Control -push-pull -demist (short) -(heater, heat+vent eqiuip, Twin
    Cam)
17H827    15.R.43   Knob -(for AHH5601 control ass'y, heater, optional, 1500)
17H827    16.R.45   Knob -(for AHH5601 control ass'y, heater, optional, 1600 and
    1600-MK-II)
17H827    16.R.45   Knob -(for AHH5602 control ass'y, heater, optional, 1600 and
    1600-MK-II)
17H1683   TC.R.56   Knob -(air control, heater, heat+vent eqiuip, Twin Cam)
17H1683   TC.R.56   Knob -(demist control, heater, heat+vent eqiuip, Twin Cam)
17H1683   TC.R.57   Knob -push-pull (air) -(push-pull air, fresh air vent equip, Twin Cam)

17H827   Knob -Heat and Vent Control -(1500, 1600, 1600-MK-II)
17H1683 Knob -Heat and Vent Control -(Twin Cam)

I believe this is original style for the control knob for the Air and Demist controls for all MGA, Including early 1500 and all Twin Cam (correct me if I'm wrong). This is a snap-on knob, press detent pin underneath to release it from the shaft. As far as I know, the two part numbers are identical parts, just a different number assigned for introduction of the Twin Cam model.