I think it's the boost bottle you've seen as Brian said. It basically acts as a "buffer" storage area for fuel vapour unable to enter the cylinder when the reeds are closed. Rather than going back into the carb and interfering with the next fuel vapour charge, it gets pulled into the bottle, ready to go more quickly into the cylinder when the reeds next open than the fresh charge from the carb.
At least, this is the gist of what I've read on the net. It supposedly smoothes acceleration from low to mid throttle (less so at high) and improves mileage a little.
Just what I've heard, they don't seem to common these days. They are mainly used on very highly tuned racing bikes AFAIK. Anyone here know anything else about them?
Hope this helps some,