Help finding casting date on 318

Hello,
I am helping my neighbor with his 73 scamp 318. There are no casting numbers like I am used to seeing on the rest of my small blocks. There is a 4 digit code that says 1343 next to the casting numbers.

The best I can tell the assembly stamping indicates a 1973 block built in the Mound Road assembly plant the the 10,000 calendar day isn’t making sense. I do not see any VIN stampings below the passenger side motor mount either.

Any help is appreciated.

Build sequential numbers are:
3M318R C7052308

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