Hi Eugene,
Maytag switched over to the Orbital transmission in either 1988 or 1989 (according to some literature I read at Glenn's), but they still used the Helical Drive pulley like the others. So, it is somewhat of a misnomer when the earlier machines are referred to as "Helical Drive" and the later machines are referred to as simply "Orbital" or "Non-Helical".
The control panels can be misleading as well when you're trying to find out which transmission it has. The best sure-fire way to tell which is which is to open the lid, unless it has been modified by one of us, of course! If it has a Power-Fin with a wide base, then it's a slow-stroke, and if it has a narrow-base agitator with pointed fins near the base (regardless of the color; the first PowerFlex Orbital agitators were turquoise), then it's a fast-stroke. All of the machines with the ivory buttons on the control panel were slow-stroke. The first Orbital-tranny machines kept the black control panels that the last slow-stroke machines had, which is one of the reasons (besides being unattractive to begin with) why that is my least favorite panel. I prefer the center-dial machines, or if I had to have the timer on the right, it would be the 10 series.
(As a sidenote, the other washer design that was last produced in 1988 was the potato-pulley Westinghouse front-loader, when the (OY) Dual-Tumble was introduced. Those ran from 1989 to 1994, and the worst part is, like the Maytags, they look virtually the same on the outside! One of the ways you can tell, though, is by looking at the controls. If they have silver knobs, then it's a potato-pulley. If the knobs are black, and the timer knob is wider than the others, it's a Dual-Tumble.)
--Austin