Those markings can vary a little bit.
View attachment 144500
The casting number looks correct for an E-body, and the "300" date code suggests 30th day of year in 1970, which may mean this axle we not original to a 1971 model year. There may be date codes on the rear drum brake backing plates (inner-=side, medial) to confirm, as it may be unlikely they were ever changed.
The marking in your photo are getting too hard to discern top be conclusive. I think I see a "G" under the upper marking, which may by an circled X, which are just guesses.
The third member may have markings indicating the gear ratio, particularly when it deviates from a 3.23 standard.
As for the axle tube markings, you got me.