oops, my mistake! I knew what I meant It's byte 80 (starting at 0) - the ecma-119 pdf shows the values as starting from 1 (so it's byte position 81) and that's what I (badly!) tried to make clearer.
What's in the 9 blocks at the end of that ISO? Just zeros? That number is only useful if it was right in the first place (and if other file systems are present, they could make it wrong). There should be no constant overhead, no. If there was, you'd kinda expect it to be 16 sectors worth (meaning it didn't include what's before the PVD).
The 9 is probably just rounding up the image size to a multiple of 16 sectors (1 ECC block).