It took me a while, but after ample deliberations and the extensive reworking of tube parts, I got to a second state. The instrument is much more stable now, but, as were my Adolphe Sax example horns, it still is a bit quirky in its lower end, but manageably so, now.
As concluded previously, there were differences between my instrument and the Adolphe Sax examples is its upper and lower ends. Both the necks and the bottom bow and bell were reworked. The straight tube remained as it was, except for a couple of changes in tone hole sizes. The text on the bell, honouring the 25095 horn as well, now reads as follows:
à monsieur Adolphe Sax
saxophone alto en Mib à cône parabolique
perce selon les examples du 24495 & du 25095
grâce à Leo van Oostrom, musicien
Marten Postma, Yzendijke
right hand side.
left hand side.
Studying my examples, there proved to be practically no difference in taper between the 24495 and the 25095 necks, but there is a difference in length. Not only in the lengths of the necks themselves, but also in the arrangement of the entire instrument. You will remember that these horns were not build on the same moulds. The neck of the 25095 was the shorter one of the two. This I followed, reworking two of the necks in such a way as that I got the same kind of difference in length. Moreover I reshaped the entrance of one of these necks to reduce its entrance resistance.
The bow was more or less correct in general shape, but a bit 'bumpy' in profile. This was reworked in order to achieve a much more smooth development of the form. Moreover, two of the tone holes were shifted and enlarged to amend intonation problems. The bell was more of a problem. Various options were studied, streching the profiles of the 24495 and the 25095 in different manners to reach to low-Bb. Yet another cardboard bell was made. Comparing the results of this scaling and the cardboard cone with the profile of the gray bell of the first state, two new bells were now made, the widest one of which worked best and was completed to the final state. You will note that this bell is also quite a bit longer than the gray bell once was, amending intonation of a rather sharp low-Bb. Both tone holes were also redimensioned.
I think it worked out well. The instrument feels a lot more like its example horns, which was the primary goal, of course. As with these horns, there is quite a blowing resistance, but the growing resistance it once had progressively from Eb downward is a lot more agreeable now and given a nice reed you can make a low-Bb just like you want to. Intonation has improved. The necks work both fine in this respect and give hardly any of the nasty sharpness in the upper range that saxophones so frequently suffer from. As for sonority, some people mentioned a certain likeness to the oboe. For the altissimo register fingerings are sometimes the same and sometimes not, but the notes are always there. Some multiphonics work while others don't or produce another sound – especially when fingerings stretch over the entire length of the tube.