My guess is that they can configure the sump for a variety of capacities and/or there are some 603.96x applications that require installation of the side chamber after the engine is installed.

There is only one main section of pan in the 603.96x and it's an aluminum piece. Not like the 617 that has an aluminum skirt and an iron sump.

The 603.97x supposedly doesn't have the removeable side chamber.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD