This is also something of a red herring. I mention it because I'm trying to be complete and because the end of the Series Ia horns is one of the theoretical possible last dates of production for the "parabolic bore".
Briefly, the bore of the Buescher saxophones, (possibly) up to, but not including the "Big B" Aristocrats and 400's, are "closely" based on the original Adolphe Sax design, which is supposed to be a "parabolic cone". To check whether your horn has a parabolic cone for a bore, you can hold the horn up to the light, look into the neck and see if the tone holes "disappear".
Ok. Sounds good. Unfortunately, you can also see this same phenomenon if you've got a horn with a bent body tube.
There is a really, really good, in-depth discussion about the "parabolic bore" at: http://www.saxontheweb.net/vbulletin/showthread.php?t=35680 and http://www.saxontheweb.net/vbulletin/showthread.php?t=77 -- the latter thread has discussion from noted saxophone authors Paul Lindemeyer, saxtek and Paul Cohen.
The conclusion is that a "parabolic bore" probably doesn't really exist. At the very least, it hasn't been proven to exist.
Summary from Dr. Cohen:
Whatever difference the parabolic cone (theorectially) makes in the sound pales in comparison to the mammoth difference in tone, pitch and response small changes to the neck and upper body bore dimensions create. Trying to find and define the mythical parabola misses the huge point of other far more signficant delineators and influences.