Andy -
The only WP/KM BD agitator that comes to mind having metal parts in it other than the original DA is the Vari-Flex, which also has a metal spring.
The Double-Duty Surgilator to my knowledge doesn't have any metal parts either in-molded or added as an assembly, and it is a one-piece agitator like the rest. To be very knit-picky, most of the single piece agitators aren't truly one piece designs, as many had separately molded transparent snap-in rings underneath, inside near the base of the agitator. I have never been sure why these are there, but they seem to be anchored at and cover up molding injection points which are sometimes called sprues or gates in plastics molding. The Penta-Swirl, most later Roto-Swirls, and the Penta-Vanes have these, in fact the Penta-Swirls are notorious for gathering and suspending lots of lint from that snap ring, almost like a mini-clothes line. Just a tid-bit of trivia there...
alr2903 - if I recall, our Moms had the same 1974 Kenmore? You are very correct, their capacity was amazing, and still is I think. Our standard dryer back then would really fill-up if we washed a big load. The Penta-Vane does not mess around, and creates lots of turbulence. Its not quite as refined as the Penta-Swirl perhaps, but it can handle big loads I think a little better.
Gordon