Slant 6 engine boss decoding

Hey folks, curious as to when my 225 was produced. A lot of online searching led me to write down the numbers behind the coil pack. What I saw (picture attached) is 6225R 2 5 28. I cant seem to find any kind of breakdown as to what these mean. All I know is the 6225 means it's a 225.

According to the 10,000 day calendar this engine was built Saturday June 29th 1968.. that's if the 2528 is the date of manufacture.

I guess my reason for wanting to know is in 1976 they went from the forged crank to the nodular iron crank. The door tag in the car says the car itself was produced 6/1976 so I would think that would give the engine an earlier 76 production date.

Any help is greatly appreciated!

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