Constant HEADER_PREFIX 

const HEADER_PREFIX: &str = "atune-parity/";

The header without its version.

The parser looks for this, so a block from an arm that has moved to atune-parity/2 is found and reported as a header mismatch rather than as a missing block. A format change is then a visible version bump, which is the whole reason the version is in the text (§6.2).