At first, Ribs in the human body and Blackbirds in a pie (from rhyme) pointed in a few different directions, so the board still felt wider than one exact category. One tempting read was "a broader umbrella topic". Ribs in the human body, Blackbirds in a pie (from rhyme) can point toward several nearby themes before one clue makes the exact category level visible.
Blackbirds in a pie (from rhyme) is the clue that narrows the board into one category with clearer boundaries.
Another nearby read was "a one-clue surface theme". Early clues often tempt you to overfit the board around one obvious surface similarity. The final answer works because it keeps every clue at the same level of specificity, not because one clue happens to fit first.
Once I read the set through a category board focused on things, examples like "Ribs in the human body" and "Blackbirds in a pie (from rhyme)" stopped feeling loose and started landing cleanly.
Karats in pure gold, Letters in the Greek alphabet, Hours in a day keep the clues at the same category level, which is what makes the board feel like one exact set instead of a broad umbrella theme.
The answer was "Things that come in 24 parts or units". More precisely, the board resolves as one concrete category with members that stay at the same level of specificity as Things that come in 24 parts or units, which is why "Things that come in 24 parts or units" fits better than "a broader umbrella topic" or "a one-clue surface theme" once the full set is checked.