Bit larger than an overgrown Anderson shelter:
Your box in the 24 Jun pictures is from 1965 or later and contained US M1 design 105mm Howitzer High Explosive shells. These were fired from the 105 mm Pack Howitzer L5 (In service 1961 until the mid-1970's).
The gun was a modified Italian OTO-Melara Mod 56 and was also used by the Australians. The box you have appears to be of Australian origin, possibly from the Maribyrnong munitions facility.
Did the Jersey TA used this gun? If so probably the box was "recycled" for some other use. Failing that it came from a junk shop!