John,

In big letters, up at the top: G4040ML1200CU. And in the lower corner, Class CTL. Those first few numbers and letters say it has 40 spaces, in which 40 poles are permitted. Class CTL means that it's designed to reject Class CTL tandems, which means that they've either modified the panel to take the tandems, or they're using non-Class CTL breakers.

Also, assuming that this is a "Lighting and Small Appliance Panelboard", the absolute maximum number of poles permitted under the NEC is 42.