From 1956-May 1986, the lower housings were cut for an o-ring about an inch and a half down the top center tube. 

The upper trans housing for the 12 series machines (1984-July/Aug 1989) had a unique design that only fits those years. They are not backwards compatible with previous years/generations.

In June 1986, Maytag switched to the lip seal at the bottom of the trans which was used into and throughout the entire orbital series until 2006 when the dependable care machines came to a production end.

You can interchange the upper housing within the 12 series line. My guess is that you're trying to acquire a lower transmission housing that accepts the newer lip seal design? It's possible as long as your watch the date codes from a donor machine and another trans has not been swapped in at some point during it's life.

Is the upper trans housing for the 12 series machines better than the older design?

It's not better, just different. Instead of having clips and washers hold all of the lower gearing in place, the 12 series incorporated a machined....for the lack of better term...."notch" in the upper housing that accomplished this. With the pre 12 series machines, one can remove the upper housing, turn the lower transmission housing upside-down in an oil pan and drain the contents without the gears falling out. In the 12 series machines, since the upper housing holds the gears in place, one must either remove the gears or hold the pitman gear (which hold the rest of the gearing in place) while tipping the trans to drain. Otherwise, the gears come crashing out. I assume this was done to facilitate quicker assembly while using less parts. As I stated above, one design is not superior to the other, just a different approach.

I have found that the neoprene o-rings seem to hold up better than the lip seal. However, the lip seal is much quicker and easier to replace. To be honest though, by the time the lip seal is leaking, it's time to change the oil. If too much oil leaks out, it needs to be added or, better yet, changed. You could experiment by using  a fluorocarbon (viton) o-ring which should be superior to the neoprene. I see the lip seal becoming very difficult to acquire in the not so distant future, so keep that in mind. The neoprene o-ring size is easy and cheap to come by, no reason to purchase it through Whirlpool.

I haven't opened up the A612, Sep 1984 yet.

The 612 will look identical, it just has an o-ring down the tube of the trans. You can swap both of the upper housings with zero problems.

Your attached photo is a pre 12 series machine trans?

Yes, early 70's.

I put an arrow of the machined notch which holds the pitman gear in place, thus holding the entire gear train, although I'm sure you are aware of what I'm talking about. Pre 12 series washers (pre 1984) do not have it.

Here's the o-ring dimensions:

Best to do it while they're still available (obsoleted quite some time ago). The brass washer takes the beating but I've seen a few lug washers that were scored up from heavy use.

Would adding an extra spring to the motor carriage help?

Not recommended, adding a 3rd spring is generally a band aid for thick oil/seizing upper transmission shaft or other issues/maladjustments. Fix the problem. It also causes the tub to spin up and accelerate too fast during the spin cycle making it more prone to tripping the out of balance/lid switch.