David - That's a Penta-Vane agitator, a large capacity version of the straight-vane essentially. It was used 1973-1986. It is a bit more 'refined' than a straight-vane, basically not as splashy, or as much like spanking your laundry with a ping-pong paddle as the straight-vane is.
Retropia - The early to mid 70s had feature-laden models. Yes, for a 500 series the dryers were traditionally timed-dry only, but this one does indeed have wrinkle-guard and auto-termination dry. The dryer up from this one had a full-width door and an electronic sensor, but this is a very nice pair for a 500.
That was rectified the next year, as the 60/600 series was far less than these - timed dry only, and a one-speed washer.
Gordon