The extra key below the low D key means that it is a LeBlanc Rationale.  The Rationale key system is unique in that it is a true Boehm system (eliminating undervented keys), which evens out the tone, intonation and response, and it provides a simple means of shifting the scale a half step using th F# key.  They are excellent, rare, and highly sought-after top line horns made by Beaugnier  for LeBlanc.  

Here's a link to the most complete information you're likely to find on the Rationale:

http://www.saxgourmet.com/leblanc.htm

You can also go onto youtube and find demonstrations of what the Rationale system is all about.

To get an idea of value to the seller, check recent closed listings for similar horns on ebay.  One of these horns in top condition should fetch some good $ from someone who knows what they are about.