The barrel diameter difference is tiny (as in thousandths of a inch), all the R or SP heads are the same (i.e. there are essentially only 4 '21 / '28 heads, front and rear both in R and SP form).

The head will have either KV3L or KV3H embossed on top (it's part of the casting, it's not just stamped).
The barrel will have either L1, L2, L3, H1, H2, or H3 on it at the bottom on the other side to the powervalve mech.

The H means it's from an R model
The L means it's from an SP

The SP head has a smaller squish area for higher compression, this is not suited to normal road use and 95ron fuel with an R barrel.
The SP barrel is taller to put the squish back to normal again on the Se and SP models.
It was just a trick to give Honda higher compression for the racebikes with std parts.

Check both the head and barrel are the same, that's what matters, not if it's SP, R, 21, 28, A, B or whatever, they all fit and work.
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