all alternators are technically dual fields, because to get a full magnetic field requires both poles or fields ( + and - ). They are simply called single or dual fields just because the fields are wired or not.

single fields get the - field by a grounding brush straight to the case and the + from reg which is sourced from ign switch ( so, wired ) to one isolated brush. You'll find on them just one field wire

dual fields get both fields wired... being the + constant from ign key, and - feeded from reg. these require both brushes isolated, hence they get two wires and tipically called "dual field "

69 is the crossover year from the single to dual field setup. Mostly of Mopars were still single field but some high equipped C bodies began to get dual field on late 69. Then 70 become standard dual field setups on all mopars.

72 as mentioned the alts changed from roundback alts to squareback alts.

dual or single fields, round or squarebacks have nothing to do with the output itself, but theway how they worked and easier to be serviced... Dual fields are more efficient just because they got electronic regulators which are more efficient on how they work.

Squarebacks specs and advantages, are exposed on the link mattax posted