D7 power unit info wanted

I just acquired this power unit and have got it running and really runs good. For the life of me I am unable to see the Serial #  that should be on the left back side of the block. If it is there it is behind the intake manifold with no way to see it.
From what I can tell it is a military unit due to the # on the radiator grill and its colors ?
When I have been ordering parts, oil and fuel filters and such I have just been using a low # D7  3T  info. Is there a way to figure the serial # out with the rebuild tag it has on it?
I should mention it has a 60 KW generator on it.

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Old Magnet info - s/n 9J2001 to 9J3000, 2S1 to 2S5000 and 6S5001 to 6S9504

Thank you, This makes sense because I have to run it at 1000 rpms to get the correct voltage, and that falls into the serial #s that you have provided.
Again thanks so much.
Clayton