Indeed the number pressed into the lid, e.g. P59 identifies the box.
P59 was used for 25-pr. Q.F. gun: H.E, Chem. and Smoke. With different markings indicating the various contents of 4 'rounds' 25-pr.
But the same box was also used for No. 68 grenade (17 grenades in the box) or 24 Bombs, hand incendiary 1 1/4 lb.
For more information on ammo boxes see the RAOC Statistiocs 1943 Pamphlet No. 2: Weights and measurements of explosives.
Rob
PS I do thios pamphlet, and many others, as a reprint: see
www.robvanmeel.nl