[kovidgoyal]

There's a couple of things I don't understand in the secondary index record, do you? This record is apparently intended to help decode the index entries in the primary record, when there are entries with two control bytes. The list of index entries is:

Index Entries (5 entries):
Index Entry(index=author, entry_type=null, flags=0, length=1, byte_size=3)
    [unknown, unknown, tag type from TAGX in primary index header] : [0, 0, 73]

Index Entry(index=caption, entry_type=null, flags=0, length=1, byte_size=3)
    [unknown, unknown, tag type from TAGX in primary index header] : [0, 0, 72]

Index Entry(index=credit, entry_type=null, flags=0, length=1, byte_size=3)
    [unknown, unknown, tag type from TAGX in primary index header] : [0, 0, 71]

Index Entry(index=description, entry_type=null, flags=0, length=1, byte_size=3)
    [unknown, unknown, tag type from TAGX in primary index header] : [0, 0, 70]

Index Entry(index=mastheadImage, entry_type=null, flags=0, length=1, byte_size=3)
    [unknown, unknown, tag type from TAGX in primary index header] : [5, 0, 69]

Each index entry has three values. The third value is a tag number from the TAGX block of the primary index record. For example 69=mastheadImage and indeed, the tag with value 69 is used to point to the offset of the record containing the mast head image in the periodical index entry.

However, I do not understand the purpose of the first and second values. The second value appears to always be zero, but the first is always zero except for the mastheadImage entry which is 5. Do you know what the 5 means? It could just be the count of the index entries, since there are 5 index entries, but I don't really see the point of that.