* Possibly the most important: in 1921, Buescher patented a new invention called the "Snap-On Pad". This innovation is important as it is considered the first real "resonator" (unless you want to consider the Conn Res-O-Pad a resonator). Take a moment to read my detailed feature page on this innovation.
* In 1928, Buescher introduced three "gimmicky" horns that were actually a significant improvement over their older, conventional siblings: the Straight Alto and Tipped Bell Bb and C sopranos. Take a moment to read my feature page on these horns.
* In 1931, the waning days of Vaudeville, Buescher was contracted to create, of all things, a straight baritone: a one-off working custom horn. Take a moment to read my feature page on this horn AND a modern straight baritone.
* Baritones switched to the "rounded rectangle" G# key a little after s/n 173000 -- and I do mean "a little after", as I have pics of a 173xxx bari with the new G# and a 1730xx bari without. There does not appear to be any other significant change in the baritone design until the Aristocrat -- excepting, of course, the Custom Built series. I will therefore "break down" the baritone models according to this break, rather than the 200xxx break.
* The Custom Built True Tone model (yes, it's engraved that) is possibly baritone only. It's sort of a combination of the New Aristocrat and True Tone styles.
* I see no change at all in design of the bass, other than with engraving, so I'll break diown these horns as I have with the altos and tenors. Do note that these horns have a keyed range only to altissimo Eb and do not have a G# trill.
* One topic of much debate is the introduction of the front-mounted altissimo F key. While it seems that ALL horns had this feature by s/n 200xxx (probably excepting the C soprano), it seems to have been introduced as an option around s/n 157xxx: and, according to Bootman on the SOTWF, there was, at least, a provision for this keywork a little earlier (probably 1923) -- and a front F from a later horn can fit on these horns, if it doesn't have one.
* There is one major keywork change to look for on the straight Bb soprano (only. The straight C soprano doesn't change): the G# hinge changes from being "under" the left-hand altissimo keywork to being in a "normal" location. (This change is very apparent in the example pics linked in the previous sentence.) I don't have enough examples to give you an exact "fail-over" date, but an 86,466 horn has the old hinge and a 137,828 horn doesn't, so we're talking 1921-1924 when this change took place. My money's on 1923.
Note that considering the C soprano never changed from the old-style hinge, this is a sure-fire way of determining if the horn you have is a Bb or C soprano.
* There doesn't appear to be much consistency in the design of the Eb sopranino until after s/n 117,230 (more probably, sometime in 1923). I can't say that later models are significantly better than earlier ones, but they definitely have tweaked keywork.
* Speaking of Eb sopranino keywork, a lot of people point out that the majority (if not all) of Buescher sopraninos after around s/n 117,230 don't have a G# key (earlier horns had either a pearl button or a rectangular bar). They're right. Foreshadowing the "articulated" G# cluster, to sound G# on the sopraninos that "don't" have a G# key, you depress the C# key. Note, also, that sopraninos don't have a G# trill key.
* I've seen a few random curved Bb sopranos that had a rectangular G# key, starting around s/n 164xxx and ending before s/n 170xxx. Again, this is not a consistent design change so I cannot accurately track it.
* Please note that Buescher did not design a new sopranino, soprano or bass after s/n 260xxx or so: in other words, there aren't Aristocrat or 400 models of these pitches. HOWEVER, if you wanted a bass in 1950, say, Buescher would trot out the old True Tone molds and make you one, generally with different engraving and sometimes with slightly different keywork, such as a redesigned G# cluster. Extended keywork ranges are NOT found, though.