That’s what I meant by my OCD not letting me buy it. If I know it doesn’t match the rest, it would annoy me!
As for why S/M halves exist, I believe there was a very, VERY brief window in the S3 era where Apple shipped all 3 pieces. That, or it’s reserved as a replacement. Real quick, can someone with an Hermes model try contacting Apple about ordering a replacement Hermès sport band? I’d be curious to know how much that costs, and if it includes the S/M. More specifically, I wonder if they can offer you just the S/M half.
(And if you can order a replacement for $50 like @Pitch’s ceramic pin, I’ll be interested in buying it!)