PART NO.  SPL.PAGE  DESCRIPTION                                                           
ACH5703   15.H.3    Plug -brake-drum (large) (2) -(for front drum brakes)
ACH5703   15.K.2    Plug for brake drum (large) (2) -(rear disc wheels, 1500)
ACH5703   16.H.3    Plug -brake drum (large) (2) -(rear disc wheels, 1600 and 1600-MK-II)
ACH5704   15.H.3    Plug -brake-drum (small) (2) -(for front drum brakes)
ACH5704   15.K.2    Plug for brake drum (small) (2) -(disc wheels only at rear)
ACH5704   16.H.3    Plug for brake drum (small) (2) -(rear disc wheels, 1600 and 1600-MK-II)

MGA brake drums for disc wheel cars had two different hole sizes for the brake adjuster access holes. These nylon funnel tubes (called "plug") fit between the adjuster access hole and the steel disc wheel, allowing screwdriver access for brake adjustment (assuming the wheel is installed in the matching orientation). Not sure about the originally intended function, but they may have been intended to keep road debris out of the brake drum. For the most part these "plugs" are of interest for concours show only. Most disc wheel (or alternate bolt-on wheel) MGA have been driving for decades without these plugs in place, and I have never heard any report of any detrimental effect from them being missing.