All front drums would be swedged on from the factory.
As to why, as stated, it makes them a unit rather than separate pieces easier to assemble. Might also get balanced as a unit and that keeps it together for the future. Requires you to pull the hub and inspect and hopefully clean and re-lube the front bearings on a brake shoe replacement.
The rears would not. If they were swedged to the axle there would be no way to tighten the axle retaining bolts
The fact that one side is not points to the drum being replaced at some point in the cars life.
Or
If the cars don't have left hand lug studs on the left hand side of the car, ( up to a certain year, 70 something) there is your answer right there, someone changed the studs on the left hand side only to right hand studs.
Also I suspect replacement drums back in the day came with the hub and studs intact. So maybe a combo of the two.
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