depends on where the partial set came from. there were full same-color sets created with specific compatibility for hhkb's, 87u's, and 660c's, and then there were a few partial left-over sets with varying degrees of compatibility.
so you'll want to ask specifically for an 87u set to ensure compatibility, otherwise you'll have various gaps (no long right shift, or incorrect/missing bottom row keycaps, etc.) if you get another board's set.