The first 20 bytes of the block header format is the same for all types of blocks.
The ID of this block.
The ID of the root block of this tree.
The ID of the next block in the chain (at same level as this one).
The 32-bit time/date when this block was last modified.
Two-byte length of data in block, including header.
Block level; 0 is leaf.
Block type, one of:
Start of data for SEQ-TYP blocks. Data spans to blk:end.
Start of data for other block types. Data spans to blk:end.