How to ID the rear axle?

In another thread I discovered that our Valiant has upgraded 8 1/4 diff and 11” rear drum brakes.
HOW CAN I ID A Mopar AXLE to learn if it was factory installed, or more likely, someone upgraded from a donor mobile? And if the latter then what model was it most like to have come from? Any place serial numbers engraved? Or other ways to ID the intended model?

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What year valiant? 73-76?

Jake