Actually, we are talking about the 722.3/.4/.5 transmissions.
Steve is correct in that the outer sealing ring was modified from a flat ring to a "T" ring, but there are other important modifications as well. The early style B2 piston's mold was changed to beef it up to reduce the potential for breakage. The sleeve in which the B2 piston operates was changed from metal to plastic to reduce the possibility of binding (and it is imperative that this sleeve be changed). [It is important to note that this is an important purpose for the improved "T" ring: there is more surface area to contact the outer bore, but even more importantly, the outer parts of the "T" seal effectively preclude piston-to-bore contact which could result in binding, not to mention wear of the outer bore's surface.] Finally, the release clearance specificaton for the B2 piston was changed by the factory to reduce delayed engagement complaints.
The B2's design was improved in the late 80s, so there are still a lot of MBs running around with the old-style B2. And, the addition of the "T" sealing ring purportedly did not occur until late 93/early 94 but as I am writing this, I seem to remember that the 1990 722.3 transmission that I rebuilt this summer had the "T" seal, though it could have been changed due to shift complaints sometime in its life.
Todd (DOH!), the reason for my listing of specific transmissions in the FAQ as being 722.XXX was simply an artifact at that time of wanting to get the FAQ online as quickly as possible for the benefit of others -- I had forgotten that this had never been changed. Though the clock FAQ
that I produced with Jim Mahaffey has probably been useful to greater numbers of people, I am particularly proud that the B2 FAQ has saved dozens of MB owners many thousands of dollars in needless transmission overhauls. I cannot tell you how many individuals have e-mailed me to thank us for providing that FAQ -- which saved them from purchasing a rebuilt transmission. Note that the B2 piston is ~$100, so for less than $500 certainly, and likely $300 or so, your transmission is working properly again in most cases.
Related to the above, I would mildly disagree with Steve on the notion that one should not worry about it if the car is shifting fine into 4th gear and that it would be included in any competent rebuild. I certainly understand Steve's point, but we are talking about a potential catastrophic failure here that will absolutely leave you stranded if/when it happens -- and it may not be at a convenient time. And -- you never know when it will happen. I was lucky -- mine happened as I was leaving work and I was on a quiet residential street. Absolutely no symptoms until failure either.
I would strongly recommend preventative maintenance on this item if you have one of the affected cars and that would be roughly any pre-94 MB for the "T" sealing ring and pre-87-89 for the sturdier B2 piston. Note that if you have a catastrophic failure with a B2 -- and you are away from your trusted shop -- it is very unlikely that a shop will replace the B2 piston alone, particularly if it is a non-MB transmission shop. Nonetheless, I would recommend that you at least print out the FAQ
discussed and keep it in your car. This way, if you haven't replaced the B2 and it fails, a shop might be more willing to do the replacement for you if it happens.
Steve's point about competent shops is also well-taken, and is great advice for someone that does not do their own transmission work. We know that Steve does great work and is very knowledgeable, but there are shops out there that will take your money and not give you what you need or want -- in this case all of the MB upgrades in your transmission rebuild as described below.
Just as one example, MB does lots of upgrades on their components through the life of the cars, as illustrated by the B2 discussion above. Another important upgrade on an MB rebuild is to include the automatically-adjusting B1 piston which was phased in during production. How many shops know that this is even an option? Not many if they are not MB-specific. Related, how many 190 transmissions have been rebuilt without the inclusion of a less-than-$10 part that will prevent catastrophic failure of that transmission? [You'll know that you're in trouble when your 190 locks in reverse.]
Many participants in these on-line forums like to save a buck, but along with this objective must come the knowledge to know when saving money is truly the wisest path to follow. Bottom line is that you get what you pay for -- if someone quotes you $500 -- 1000 to "rebuild" your MB transmission, well, don't say I didn't warn you. Have shortcuts been taken? Do the math: ~11 hours flat rate for the rebuild (not including R&R) and the B2 is around $100, not including everything else -- including profit. Do yourself and your MB a huge favor and patronize a shop and individual like Steve who knows these cars and what is required to maintain them properly.