5.5-in ammunition box?

[tankbarrell]

Further to Ed's explanations, WM 108 is the propellant code, Waltham Modified and the size of tubular grain.

Pg/C is not an abbreviation for packing but the packers code, Pickering Filling Factory of Ontario followed by /C for Canada.

Mk 1 FOIL is just that, a piece of tin foil, crumpled up in the charge bag to prevent coppering of the rifling. As it burns it combines with the copper residue from the drive band and forms a brittle substance that reduces the fouling.