Confirmed, the Losi Smart diff internals fit in the HD diff case. Also I checked the Smart Diff parts list and the part numbers are the same (Smart diff case and HD diff case). But it really doesn't save you any money unless you reuse your ring or spur gear. Because the individual parts just about equal the cost of a compete Smart diff.
Here are the part #'s for the individual internal parts, and you will also need the internal springs as well. The one that is covered is part #3604.
Everything fits (Smart diff internal parts into the HD diff case).