You have a complete box, so hang onto it. They just don't seem to be making any more 1942 ammo these days, so the price is bound to go upward sooner or later. I remember this stuff at $4 a box and people complaining about the high price.
The DI headstamp is the factory: Defence Industries. This was a government-owned Crown corporation which was set up by the people from the Dominion Cartridge Company and the staff was trained by DC Co people. ALL Defence Industries ammo uses the .210" Boxer primer, it ALL was non-corrosive and non-mercuric. It was superb ammunition when it was made but, as your friend previosuly noted, even the best can suffer from poor storage.
They headstamped only SOME of their 1942 production like this, Drachenblut. Part way through the year they changed to the 'late '42' headstamp, which they kept through the remainder of the War: just the letters 'DI' for the plant, 'Z' for the powder (they used a Canadian-made powder modelled after Nobel Neonite; it was quite similar to IMR 4895) and the 4 figures for the date.
Your EARLY 1942 ammo also has the Mark of the cartridge on it; that is what the 'VII' is for. They are Mark VII (Ball) cartridges, using the 174-grain composite bullet: drawn plated mild-steel or gilding-metal jackets, aluminum filler plug to rebalance the projectile in the nose and a filling of about 98 lead and 2 antimony.
Later, they changed to the later system, which meant that you couldn't tell what type of ammo it was by looking at the base of the cartridge for the correct identifying letter: W for AP, G and a figure for Tracer, B for Incendiary (comes from Buckingham, which were introduced in War One for shooting down Zeppelins: work fine on Messerschmitts, too), L for Blank and so forth. Instead, they used colour-coded bullet tips and kept the casings all the same, as in American practice.
But yours are headstamped 'VII', so they are Ball rounds.
Dominion Arsenals (the Government plant) followed British practice throughout the War, producing ammo with 'proper' headstamps...... but Defence Industries made a heck of a lot more ammo.
The DI brass, I find, is as good as anything made today, even if it is getting brittle with age by now. So, being that we all basically are Scots (either by ancestry or by inclination) we anneal our brass and keep on using it. BTW, all DI brass I have encountered has had rims either right AT the specified maximum of .063" thickness..... or so close to it that my Moore and Wright micrometer has trouble telling the difference. It was REALLY decent stuff.
Canadian Inspectors passed on 4 billion rounds of .303" ammunition during the Second World War and it is still turning up and guys are still banging it off. In the last year, I have picked up 1,000 rounds of the stuff at one rifle range, and that's really nice: when you have 25 rifles that think the stuff is candy, it pays to have a bit around!
Havin' fun: what it's all about.