Here's a link to the article musicalmason refers to:
http://www.dallasmusic.org/gearhead/Benge.html
Tom Turner is also a great source of things 'Benge'.
I can add bits and pieces mainly gleaned from an interest in Chicago Benges rather than as a collector or player. The earliest serial number was thought to have been 525 but 520 turned up recently on e-bay..quite beaten up and with a lot of interest shown. Believe it was pulled before the auction finished. One thing that I gleaned from following the early Chicago's was that they were probably all custom made to order. Since lots of the guys playing at the time were using the French Besson and Elden was trying to make a better mousetrap (hence the forward facing second valve slide) some also have the 3rd slide ring suspended under the slide rather than on top. The very early ones don't have the cute 'notes in a staff' engraving on the bell (believe that came in around 900-but may be wrong). That together with what's on the link above just about exhausts my knowledge.
Tom or somebody else can probably add lots more.
Regards,
Trevor