Iirc, some 15,000 rounds of AP were issued to the 3.7” heavy AA guns emplaced in the invasion areas along with large stocks of HE for direct fire missions.
Preparations were made by all A.A. defences to assume a secondary ground defence;
Bofors were provided with A/T ammunition, and sited to cover approaches to aerodromes,
V.Ps, &c. Certain 3.7-inch guns suitably sited were given an anti-ship role and preparations
were made for barrages to be put on certain beaches. Under the immediate threat of invasion
in May 1940, mobile columns of A.A. troops were formed, but these troops reverted to their
A.A. role before the Battle of Britain began"
AIR CHIEF MARSHAL SIR HUGH C. T. DOWDING... Chelmsford 20/8/1941