20 round boxes
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Some of the "Premium" ammo came in 10 round boxes. The 20 round box seems to be the standard more in North America.
Some of the British calibres came in 10 round, and even 5 round boxes, and many of the European manufacturers had smaller than 20 round boxes.
Military packaging is a bit different than hunting. For example, the .303 came in 20, 32, and 48 round boxes quite commonly. Now, I can see the 20 round box ( a Lee Enfield holds 10 rounds ), a 32 round box ( Bren LMG) but the 48 is a bit odd ( a Lewis Gun takes 47 rounds ) but maybe it is a bit of making use of the box size. Maybe it was intended to fill 1 and 1/2 Bren gun magazines.
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