Hello SOTSDO;
I am a new member and just read your very informative post regarding vintage bass clarinets. As yourself, I am a woodwinds doubler. However, I don't have a lot of experience with the bass clarinet. I did purchase a vintage bass clarinet a few years ago, but did not have the occasion to perform with it until recently. I would really appreciate your experience and knowledge in helping me to determine as much information about this bass clarinet as possible.
To provide you some information about this bass clarinet, it is a wood bass clarinet marked "Henri Leduc Paris" on the upper joint, with the serial number "2007" on both the upper and lower joints. There are no other markings on either joint, or the bell. It plays down to low Eb. This bass clarinet also has two register keys, one on the body, and one on the neck. It also appears to have the long rod (as you mentioned in your post) linking the upper and lower joints and which actives the "on-neck" register key when playing the from third line D on the staff to fourth space E and remains open on up this register. In reading your post, I couldn't understand whether you were saying that this long rod linking both joints was a good or bad thing. But this is when the bass clarinet begins to play flat. The intonation below the point where the on-neck register key is activated seems to be fine, but because it doesn't have a tunable neck that could be adjusted, it is difficult to play in tune in the upper register.
I haven't been able to find much information at all about this bass clarinet. It appears that "Henri Ledoux" may have been a stencil made in Paris, but I couldn't find anything regarding "Henri Leduc", which is probably just another obscure stencil. It does appear to be a well-made bass clarinet with nice crack-free wood, well-manufactured keywork (the finish is still excellent), and a nice sound. I am not sure what you mean by the "fork Eb" that you also mentioned in your post, so I am also not sure if this bass clarinet has this. Since I don't expect to be playing a lot of bass clarinet, I seems to be suitable for my needs and I am fairly happy with it. But if I could correct the intonation a little, I would be a lot happier!
I am wondering if this bass clarinet is possibly a Leblanc stencil. I can only guess that it was made sometime in the 1930's - 1940's, but I wouldn't be surprised if was older or even newer. (It has a fairly new case so I can't use that as a clue.) Anyway, if you might have any information, ideas, or suggestions regarding this vintage bass clarinet, I would really appreciate it! If you wish, I could provide some photos it that would help!
Best Regards, WindsDoubler