The 'AC' is one of the complicated RA tactical signs. I assume that it indicates the third gun of 'A' Troop. This site is quite handy :- Organisation of a Field Regiment 1940 5 over 8 is simpler. It's a bridging plate. It shows the tonnage rating of bridges necessary when solo and when towing. Although they were coming into use by 1940, they don't seem to have been standardised.