c4d_objects/description/oprimitiveslice.hParameters are accessed with cinema::C4DAtom::GetParameter() and cinema::C4DAtom::SetParameter(). See Parameters.
Parameter: Slice
PRIM_SLICEParameter: From
PRIM_SLICEFROMParameter: To
PRIM_SLICETOParameter: Regular Grid
PRIM_REGULARParameter: Width
PRIM_REGULARWIDTH