Right. For anyone else interested :
The AVDP is recorded in accordance with ECMA 167 and "requires an Anchor Volume Descriptor Pointer (AVDP) at sector 256...".
"An Anchor Volume Descriptor Pointer shall identify the Main Volume Descriptor Sequence..."
The Main Volume Descriptor Sequence consists of a consecutively recorded sequence of one or more Volume Structure Descriptors.
The 1st Volume Structure Descriptor is the Primary Volume Descriptor.