The 31551 is a kit to allow stacking of components inside Campbell Scientific enclosures. The kit includes two leg brackets. It is used to raise a data logger up and off the backplate so that wiring is easier or so that components can be stacked.
The 31551 does not fit inside the ENC10/12. If the ENC10/12R is used, it must be oriented with the hinge toward the top or bottom.