400AT airbox question

Neighbor has a NICE black 400AT. Not sure of the year, but I know they quit making them in 2007.

His has 167 miles on it. Has never been in any mud at all. He rode it over and asked me if I could clean the carb because it wouldn’t run without the choke on.

Found usual goop one would expect in a 10 year old machine on its first tank of gas. Had it cleaned up and running in about 30 mins (working on bikes that have never been in the mud is EASY).

So, question is, there is a big can that comes off the back of the airbox, and hangs above the right rear wheel. What is this thing? Some kind of air resevoir? I’ve never seen anything like it on any other machine.

I told him to keep stock tires on it and to baby it. He’s an older guy so it will probably be fine for him, but the 400AT is really a dog compared to the 420AT’s. That Hondamatic wasn’t cut out for 400cc’s. Add in front drums and full time 4wd and I can see why nobody wants them.