I'd still be a little hesitant to suggest that anyone could pinpoint which mowers have which trannies by year model Upgrade. As indicated above they're installed on an availability/bid price basis. The manufacturer needs X number of trannies for a given year model whoever gets the bid their tranny goes in, if they sell too many they are subject to rebid, whereas tranny may change in a given year model in any given year. Or, if they don't sell enough I'm sure they carry them over to the next year.
Likewise, it's really tough to pick out a year model on a mower even if you know what you're looking at. If you're dealing with someone who's acquired the mower second or third hand, or it was left in the tree row behind the house by the previous homeowner, they probably don't have a clue what year model it is, and you're going to have to go look at it anyhow.
Might be nice if we could find a few guys with known makes and models of transaxles installed in unaltered machines to get a few a pictures from the bottom side so you could print them out and take a picture along to look at. Or, maybe even a few pictures of factory machines they came out of so you could send a picture to someone and say, "Does it look anything like this?"
But, I do most certainly agree that we can come up with something that would be of benefit, and it is certainly a worthy topic. Personally I wouldn't have a clue what a 600 Peerless looks like, don't know what the 800 you comment on up there looks like Squidd. Tore my AP down to start building on it yesterday (yeah... I'm running a little behind! But when I get moving on it, it will go together fast, I promise. Might be next year before I get an engine, but...) that one had a three speed Foote tranny in it that looks almost identical to a Peerless 700 casewise. Don indicated they're easy to identify, Foote is the only on that stamped the shift sequence on the top of the case, even though it's buried in the mowchine. Once I scrubbed a little dirt off, sure enough it was stamped Foote on the case right below the shifter.
But there are obvious giveaways like the bushing versus bearings on the shafts of the 350 versus 400 model Peerless. I think we can make something work here pretty easily.