M21 is among the most-frequently-cited fender-tag option codes because it appears across so many builds. Bright trim that sits in the gutter of the drip rail along both sides of the roof — easy to miss on a finished car (it reads as part of the roofline) but immediately conspicuous when missing.
On a Plain-Jane Super Bee or Road Runner ordered without bright trim, M21 is absent and the drip rail is body-color. On any car with the Custom Exterior Group (A21) or higher trim, M21 is included by default. Restorations that swap base trim for upgraded trim (or vice versa) leave M21 mismatched against the tag — a common giveaway.
M21 lives in the option block, typically rows 1 or 2. It pairs visually with M25 (body-side mouldings), M26 (belt mouldings), M31 (wheel-lip mouldings), M71 (rocker panels) — together these comprise the bright-trim suite.