Steve, it's just like the Cabas Piano/Mezzo, but it's the largest of all three (hence the term "Alto")
Here: eBay: Authentic Louis Vuitton Monogram Cabas ALTO EXCELLENT (item 330070238136 end time Jan-09-07 06:07:56 PST)
The Cabas Alto is one of my favorite mono LV's. The fact that it's "convertible" is really a non-issue because when you wear it on your shoulder the bag is closed under your arm. It depends on you though... If you find yourself gravitating towards Buckets and the like then open tops don't bother you. If you gravitate towards closed top bags then you may not like the Alto.