Question: When Was A-440 Pitch Adopted?

Mandatory clarification: By Whom?

There were a few uses of A-440 as long as a couple of hundred years ago, but with most prior usage being at somewhat lower pitches.

There is a lengthy summary of pitches used in the past in Helmholtz's On the Sensations of Tone, which remains a "classic" work" on the subject. The Second Edition, originally published in 1877, is available in English translation from Dover fairly cheaply. There's enough history in it to totally bore most people.

The Helmholtz history concentrates largely on pipe organs, since they can't be easily retuned (each pipe has to be separately changed).

Helmholtz does indicate A-440 used in 1829 by the Paris Opera Orchestra, with others going from 372 to as high as 459. As he doesn't mention (that I found with a quick look) the A-440 as an adopted standard, it likely wasn't much of a standard at the time of original publication.

(Handel's personal tuning fork, ca. 1751 was reported to be at A-422.5)

Adoption of A-440 as a "physics standard" varied from one country to another over a period of at least a few decades; and musicians ignored the changes fairly widely for a few more decades. Some modern conductors prefer something else even now, with a couple of symphonies reportedly (according to player anecdotes) "tuning up" to as high as ~A-460 or higher.

Changes over a fairly wide range are not particularly problematic for string instruments, since retuning is fairly simple and is necessarily verified before each session.

Most wind instruments can be tuned down by at least a semitone using slip-joints or moving the mouthpiece out, but tuning up from the "built pitch" can be a very real problem. For this reason many (wind) instrument makers probably went to the A-440 fairly quickly, even before it was universally adopted by players, since that left the instruments usable at the lower pitches that were common during the transition and also with the rare group that wanted the higher pitch.

I can't say when A-440 could be considered universal among manufacturers, but my Martin Bb Tenor saxophone made in 1934 was easily tunable to that standard. When I played it a lot in the late 1950s, there were still lots of old (junker) pianos tuned to the "earlier standard" still in use by piano tuners who thought the new standard would "stress the woodwork" of barroom relics. The A-440 was certainly pretty universal for concert grade instruments, but the tuners still had the old forks in their toolkits, which gives some idea of how persistent the earlier (lower) pitch may have been.

John