Dug out the "field guide" on Warsaw Pact - Calibre Ammunition Box Markings. Handy little thing to have...
The Chinese tins are labeled Lot - Year - Factory. So, Lot 73 - Year 1977 - Factory 912. As for the rest of the markings, 7.62 is the calibre, and as I recall the 56 refers to the fact that this is ammunition for the Type 56 rifle (which can mean either the Chinese SKS, the Chinese variant of the AK-47/AKM, or the RPD) all of which are chambered in 7.62 x 39mm. 750 is the quantity of the can - and an easy way to distinguish between the cans of Moisin and SKS ammo (7.62 x 54R and 7.62 x 39mm).