Can I plug a HI 2151/20WC wall mount display into the HI 2151/30WC electronics?

NO.  The display for the HI 2151/20 will not work with the electronics of the HI 2151/30 unit.

We have safeguards installed to prevent customers from plugging a HI 2151/20WC wall mount display into the electronics of the HI 2151/30WC. There is a keyed connector on the front circuit board edge connector to prevent the wrong display from being installed. The front display ribbon connectors are also physically different on the newer HI 2151/30WC units. But if you do somehow manage to plug the HI 2151/20WC display into the HI 2151/30WC electronics, the HI 2151/30WC MAY be damaged. And installing a HI 2151/30WC display into the HI 2151/20WC electronics WILL damage the electronics in both components. The reason damage may occur is because the HI 2151/30WC unit uses 5 vdc to power the front display module and the HI 2151/20WC uses 10 vdc.