I've noticed that Maytag washers sometimes have a different "series" within the same model.  Is that because certain features and/or improvements were made during that model's production? 

I've been doing a little research into parts availability (timers/switches) for an A606 that I'm looking at.  The one I'm looking at has THREE fill levels.  However, when I google parts for it, the pressure switches that I'm finding all have FOUR levels. 

Just curious!

The one you are looking at with 3 water levels, were early 606 models from when these were announced in 1966 or 1967.  Friends of my parents had an early 606 with 3 water levels.  It had the washing cycle and soak cycle.  No permanent press cycle.