I guess my own hypothesis would be that in fact the horns were just built to a lower pitch, and that this pitch (actually "these pitches" since there were several ranging from below 435 to about 460 as far as I can see in various historical contexts) was what ensembles used for tuning. I'm having difficulty imagining why instrument makers would build brass instruments flat with the idea their players would constantly have to "lip up" the pitch, or that they would build brass instruments flat while not doing the same with, say, woodwinds (which would be significantly more difficult to "lip up" in a consistent way).
I do have in my possession a copy of the "True-Tone" quarterly, Vol. VII, July 1909, published by Buescher which indicates that the manufacturers (at least certainly Buescher) were attempting to manufacture instruments that played to "true-tone" standards and did NOT require constant adjustments of various kinds by the player. They advertise cornets that are "Multi-Pitch Perfect in Bb and A, High and Low Pitch" and say
In these days -- when standards of Pitch differ, not only in different cities, but in different organizations and in different theatres in the same city -- it is a satisfaction to know that no matter where you may go, or in what Pitch or Key you may be called upon to play, your Cornet is immediately available for that service, and the intervals just as precise whether played in a Flat Key or a Sharp Key, in High Pitch or Low Pitch.
They offer the same capabilities in their other instruments as well, including tubas where
True-Tone Tubas are immediately available for use in any variety of Pitch, without the use of detached slides or other vest-pocket devices.
The picture of the front-action 3-valve Eb tuba in the newsletter shows a slide labeled as "Instantaneous low pitch slide", but the picture of the top-action horn does not have this. The front-action horn appears to have both that "low pitch slide" AND a tuning slide in the lead pipe, but the top-action horn (which looks pretty much identical to the 1917 one) does not have a tuning slide in the lead pipe. Unfortunately, nowhere in this issue do they given an account of what "low pitch" or "high pitch" means in terms of frequencey, but they repeatedly emphasize that their instruments are built to cover "any variety of Pitch".
Anyhow, 435 seems to have been one of the fairly popular pitches in the late nineteenth and early twentieth centuries, and was even recommended by the Austrian government at one point (how's that for government intervention?). And 440 was only adopted as a kind of informal standard in the mid/late 1920s, and only formally (in the US) in 1936. Some summaries of this sort of pitch history can be found in several Wikipedia articles, and with pointers to more reliable historical literature -- for anyone who cares. Another intestesting source is the article "History of U.S. Standard Pitch, A=440 Hz"
http://www.mmdigest.com/Archives/Digest ... 05.05.html where it's pointed out that 435 was favored for tuning pianos, by the French for a long time, and also by the American Federation of Musicians. There are also some interesting remarks on the effect of temperature in this note. However, it appears that some of the information in this article is not quite correct, such as the claim that the US "officially adopted A-440 as standard pitch in 1920" (which is inconsistent with just about every other history I have seen). There remains a certain degree of confusion concerning the very complex history of musical pitch, and it's difficult to sort out.
I wish I had been aware of all this when I started my mouthpiece searches/trials and struggles with the 1924 horn. It would have made my experiences with the pitch of that instrument considerably less surprising and given me confidence to proceed in a direction I was otherwise reluctant to.
A primary reason that I started with the third valve slide when I decided to introduce the tuba to Mr. Hacksaw was that it was clearly so long that even if I did screw up a bit it almost certainly wouldn't have made any practical difference
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