A note about Leblanc family altos. (Leblanc pro, Noblet, Vito)
Leblanc tell me that the pro design was changed about #7000 (1964) when
the 'slightly bent' neck changed to the 'crook' type. (I don't know whether some strange goings on duplicated some serial numbers but I have seen three numbers around 5000 which are of the later design! I suspect that instruments sold in the USA were renumbered.) The later neck took about 25mm off the upper joint length so the wrong neck (which I have seen!) can give a funny horn in E or D! The bore also seems to have been
enlarged a little about the same time. The earlier ones play a bit more 'reedy' than the later ones (or maybe just easier!) and have a more conventional hand position because of the
longer upper body. I have heard it said that the extra wood in the top joint of the early ones and the Noblets, makes for a better sound. Not sure I can believe that, but who am I ...
Personally I like the crook type neck better because of the more comfortable angle and higher hands. The
ones I have had all play in a very similar way. The Selmers I have are 17.0mm bore while the various Leblancs and Noblet have been around 18mm, Vito 18.25. Leblanc have the trill keys
15mm nearer to the right hand than Selmer. The Leblanc layouts are almost identical except that the Leblanc pro has the auto speaker mechanism operated from the 'A' key, while the Noblet
and Vito work from the Thumb key. The pro also has LH3 closing the Bb tonehole between LH1 and LH2 like the bridge mechanism does. I can't find a use for that unless it is around F5 in
the altissimo and then I don't play well enough to comment on its usefulness. Must be there for some reason though. Makes a very flat forked clarinet Bb or chalameau Eb.
Leblanc tell me that the pro design was changed about #7000 (1964) when
the 'slightly bent' neck changed to the 'crook' type. (I don't know whether some strange goings on duplicated some serial numbers but I have seen three numbers around 5000 which are of the later design! I suspect that instruments sold in the USA were renumbered.) The later neck took about 25mm off the upper joint length so the wrong neck (which I have seen!) can give a funny horn in E or D! The bore also seems to have been
enlarged a little about the same time. The earlier ones play a bit more 'reedy' than the later ones (or maybe just easier!) and have a more conventional hand position because of the
longer upper body. I have heard it said that the extra wood in the top joint of the early ones and the Noblets, makes for a better sound. Not sure I can believe that, but who am I ...
Personally I like the crook type neck better because of the more comfortable angle and higher hands. 
The
ones I have had all play in a very similar way. The Selmers I have are 17.0mm bore while the various Leblancs and Noblet have been around 18mm, Vito 18.25. Leblanc have the trill keys
15mm nearer to the right hand than Selmer. The Leblanc layouts are almost identical except that the Leblanc pro has the auto speaker mechanism operated from the 'A' key, while the Noblet
and Vito work from the Thumb key. The pro also has LH3 closing the Bb tonehole between LH1 and LH2 like the bridge mechanism does. I can't find a use for that unless it is around F5 in
the altissimo and then I don't play well enough to comment on its usefulness. Must be there for some reason though. Makes a very flat forked clarinet Bb or chalameau Eb.