Don't overthink this kind of thing. They don't mention days of the month, so it could easily mean 31st July 1976, and 31st August 1977 respectively. It's not a whole month's worth of production in either case, and production wasn't necessarily linear or 'On/Off' switch with either maximum capacity or nothing.
The car in question WILL have left the factory with a full production number engraved (they were not stamped) in the usual place - on the firewall, above the master cylinders. No exceptions for standard production road cars in the S30/S31 series to be sold to the general public, so something has happened to the car and it has lost its original firewall-engraved number.
I can't imagine what kind of situation would require the replacement of the firewall or a large number-bearing section of it, but - having seen all sorts of beastliness in the past - I'd be suspicious and want to investigate further. The stamped V number on the strut tower must relate to something that you can look into (via NSW Police?) and I guess it must have been checked out, but I'd want to get to the bottom of it. First thing I'd do would be to take off the cowl panel and look at the inside of the firewall above the master cylinders. I'd be curious to see what it reveals...