"The first-line pro-quality instruments were identifiable by the intertwined LL logo, and have a completely different bore size from the 300 series horns." "My own 1960's-vintage model 517 was abandoned by Leblanc in favor of a slightly smaller-bore 417 (with a concomitant loss of power and resonance). The x17 numbers indicate low-C instruments;x13 numbers denote instruments with the forked low Ab-clarion Eb fingering (in lieu of a left had Eb/Ab lever)."
-Larry Bocaner (of the National Symphony Orchestra) on this topic on the Clarinet BBoards on Leblanc Basses: http://test.woodwind.org/clarinet/BBoard/read.html?f=1&i=352157&t=352157
I believe only the later models of theirs have a double register vent, which I consider to be much preferable. Basically it vents the clarion through two different register vents, rather than just one for all of the clarion and one for the throat Bb.
The model x00 is just standard keywork, so that would presumably mean a model 500 is a large bore bass to low Eb with (thankfully, in my opinion-it's rather hard to keep in adjustment) no low forked low Eb/Ab.
If it has a register vent on the neck it probably plays well, and one without it still might. (although it might have more problems.)
So I asked the seller about it, he says there's a register key on the neck but appears to only lead to one vent. Could it still be possible that a 1969 LeBlanc LL Bass Clarinet from 1969 to have the extra benefits of a double vent register system?
You've been a big help, thanks for the info!
Unfortunately just like buying many other instruments online, I'm unable to test before I buy. I beleive the one I asked about is not what I'm looking for anymore.