/INDEX=n

    Specifies the relative key for the bucket of an indexed file.
    The /INDEX qualifier is necessary only when the index number
    information is unavailable in the bucket header. For example,
    you use this qualifier to analyze a Prolog 1 or Prolog 2 file (no
    bucket header) or a Prolog 3 file with a corrupted bucket header.
    You can also use this qualifier to override the index number in a
    Prolog 3 file bucket header.

    The number you use specifies the key. For example, /INDEX=0
    specifies that the bucket is a primary index or primary data
    bucket, and /INDEX=1 specifies that the bucket is found in the
    first alternate index structure.