Genuine HRC heads for the MC21 are the same shape as stock. Meaning the lower head still has a off set plug but differ with both volume and squish to the stock heads.

The MC28 HRC head swap is just using two stock rear heads. The heads are stockers and in no way HRC. The reason for this is when the '28 came out there where very few genuine HRC parts avaliable and as it was raced in SS/SP they were only allowed to use the stock heads.
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If I have to take the carbs off once more...