Doesn't the NWMP (whatever the new name is) still come in a 20 gallon for body lifted Broncos? Don't remember seeing it on their site before the change, but wasn't sure. And like you say, it could be an older tank.
Is yours held on by a strap Beck?
Oh, and welcome to CB too by the way! Good luck with the install. It's relatively easy in fact. For me it was actually easier putting the new one in than taking the old stock one out. Bonus!
Paul