You are correct that there are two setups.
The version with the flanges is later and as you probably already know intended to make it easier to sling load the truck on/off ships.
The flange at the rear is a simple flat sheet that is held in place by the rear axle flange nuts between the nuts and the wedge washers that stop the drive flange from rotating relative to the hub.(I'm not sure if there were longer studs on the flange equipped trucks.)
The flange at the front is deceptive. It looks like the cast hub cap with the built-in flange should just be a bolt on replacement for the earlier cast hub cap. My experience is that this is not the case.
I'm not sure whether once the flanged setup became available all trucks were equipped with it or whether the plain setup continued in production (doesn't make sense to me but possible).
Another thing for me to check in the parts books.
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