In all this, it should be kept in mind that prior to Helmholtz and his creation of his "Helmholtz resonator" idea and derivation of accurate formulas for it, there was virtually NO WAY for anyone to tune each of the notes of a scale individually and to know that they were individually precisely related to specific frequencies. Even after that development made it reasonably possible to have separate references for each note, hardly anyone actually assembled the necessary sets of instrumentation.

While a particular musician might have had one, or even two or three tuning forks, it remained necessary to adjust all the other notes "by ear" from the pitches the few notes that might have been fairly accurately matched to the one or few tuning forks.

Helmholtz in fact tabulated the actual (measured) frequencies of a number of "standard" tuning forks, and identified the artists/orchestras/luthiers who used them. The "A" forks varied over at least a range of a few dozen Hz even with the somewhat crude accuracy of his measurements when compared to what we have easily now.

Although they are "relatively stable," even carefully made and meticulously stored tuning forks do change their pitch over even fairly short time periods, and the changes can be fairly dramatic in short times for those used daily, or at least frequently, or carelessly stored. (The platinum standard for the meter was discarded years ago because it kept changing its shape and size, and they said they kept it clean?)

Because of their generally larger size, organ pipes are less susceptible to pitch changes over time, so the recent (perhaps since mid 1800s or so) measurements of their pitches made with "fairly modern instrumentation" are probably the most reliable evidence of how music sounded in the past. Tuning forks can give some indication of where individual notes on a scale were placed, but tell almost NOTHING about how the other pitches within the scale were tuned.

Headlines (in some obscure places) appeared a few years ago when someone found Ledbetter's old 12-string in a closet, and multiple scholarly papers were written about how "it proved he tuned it" in such and various ways, thus "revolutionising" our understanding of his music(?). Of course I take my mandolin out about once per year and it's always perfectly in tune, but I doubt the Ledbetter soldered the tuning knobs when he first got it as I of course did.

Part of good history, or science, is knowing when to say "I don't know," although it's certainly lots of fun to make up explanations of the wisps in the mist. It's also a good idea to decide early on "what is it about this that matters most, in order to simplify things as much as possible."

John