In dtformats/documentation/Chrome Cache File Format.asciidoc Section 3.1 at the bottom of the table it says:
Offest | Length | | Desc
48 | 52 x 8 = 208 | | Padding - Contains 0-byte values
52 x 8 is 416. It seems it should be 208 bytes long in order to provide a 256 byte header. Not sure if it should be 26 x 8 or 52 x 4.