Function header_tail_kernel 

pub fn header_tail_kernel(
    key_index: b16,
    meta: KeyMeta,
    siblings: [[b8; 32]; 4],
    top: [[b8; 32]; 3],
) -> [b8; 290]

The 290 header bytes after the signature (frame bytes 128..418).

Output index i is frame byte 128 + i:

h[..]	Frame bytes	Field
0..2	128..130	key_index, LE16
2..34	130..162	pk_hash
34..50	162..178	pk_nonce
50..66	178..194	private nonce
66..290	194..418	7 siblings: siblings (subtree), then top (cert unit)

Bytes 0..256 fill header beats 2..5; bytes 256..290 become the packer prefix. Combinational.