The US government (and many others as a result of treaties at the end of WW1) adopted the A=440 standard in 1920. The American Federation of Musicians moved quicker than Congress and adopted it in 1917. That started a shift in the designs between 1917 and 1930 as the LP standard at A=435Hz instruments were gradually redesigned to the A=440 standard. It couldn't happen over night because too many musicians were playing LP (A=435) instruments. While the difference is very small, it is not as negligible as most think. I think this may be why we perceive the intonation on LP instruments to be worse than the later A=440. I'll need far sharper knives to cut that idea clean. I just don't have the Conn laboratory at my disposal.
In fact we are trying to force LP instruments to be "a little out of tune" with themselves. That's no big deal because Albert Cooper and the promoters of A=442 are doing the same thing AGAIN! RESIST! Odd that the low notes are flat almost no matter what we do? And if we alter the barrel it all goes awry, with some up and some still down. The reasons may be more complicated but in general, LP was a different pitch standard than A=440 and the designs changed. I have many instruments, flutes and clarinets that illustrate this very well. Polycylindrical design doesn't really come into play too much in the lower register. It should be possible to get almost any clarinet in tune from top to bottom below the register break. When we are continually approaching that problem with barrel length, we might just try playing in a slightly warmer room. Air conditioning could be the prime reason we wind up barrel swapping. Lowering the temp 1degree Celsius throws the wood wind pitch as much as 2 cents flat. 5 degrees and it is way off.
On trumpets and cornets, the availability of the second set of slides for HP disappeared first, round about 1922. A 1919 Conn New Wonder that I have still had both sets, HP and LP slides. A 1922-24ish Frank Holton trumpet that I have clearly has LP stamped on the receiver. By 1925, the Conn trumpets are only A=440 and have no pitch standard marks anywhere. I think Dave LeBlanc is correct in that any reference to HP or LP is gone by 1930. It probably was gone even sooner in France where the treaties are usually signed. The French were in favor of a single standard in 1859 and readily embraced 440 because when the "standard temperature" was considered, this was extremely close to the French A=435 standard which was specified at a different temperature, and we all know that temperature and humidity are the true determinants of the pitch at the moment regardless of what was built into an instrument at the factory. Looking at the 1902 JTL catalog, it is obvious that makers knew how to build an instrument to any standard. JTL offered even customization in that regard. And the history of pitch inflation in the French opera also proves that instrument makers had to know how to make in tune instruments at continuously fluctuating pitch standards. The pitch standard changes kept them real busy;- and profitable.
Find a few other Couesnon clarinets to compare it to and see if there are other details that also appear or disappear (the shared lever post comes to mind as does the adjustment screw on the A/G# arrangement). The elaborate bell markings of the French makes also seem to have changed about the same time as the pitch standardization. The prize banners and all the other scroll that is common on JTL, Couesnon, early Kohlerts, and others seems to disappear some time in the 1920s also. So by the bell, this one is older.
Regarding the barrel length, I have found several pairs of clarinets of identical make that have big differences in barrel length. At first I thought this was some kind of shortening of the clarinet as a whole but more examples proved otherwise. When the barrel got shorter, the upper joint got longer, retaining the same distance between the reed tip and the register key port. The barrel design is what changed with the trend moving to shorter barrels. So it is that distance between the reed tip and the register port that is critical, not necessarily matching a barrel length on an example even by the same maker. I have two Henry Gunckels that illustrate this, Penzel-Mueller Pruefer, Pruefer, and Penzel Mueller that also illustrate this, and some others. If that Couesnon is later than mine, it could be the same way;- tall upper joint with shorter barrel. In that case the Pedler barrel might work just fine. If not, then seek a longer barrel until the clarinet is in tune with itself top to bottom in at least the low register. I want to get myself one of these telescoping barrels so I can use it to figure out improved barrel lengths. But given that the barrel was the first part where a conical bore was introduced, even a telescoping barrel does not give us the whole picture. I think it would be better than nothing.
While barrel swapping has its' perils, generally it is fairly easy to find a barrel that works very well with a little experimentation and you won't be able to do that until you get it playing and even which mouthpiece you choose will factor into it.
