Index of reports, 50th to 75th Congresses, 1888-1938. (OCLC #9796069)
There is a fixed field code for use when a book is an index to bibliographic material other than itself:
Cont: i
This code is not be used when the resource just contains an index to its own contents; there is a fixed field specifically for that:
Indx: 1
Naval Digest indices (1970-1979) / Allan S. Rehm [and others]. (OCLC #18772227)
I couldn’t find where we had any volumes of the title “Naval Digest”, so dug through the V schedule looking for a general Naval Science research number (leaning toward V394).
I ran this past my supervisor, who works more with serials; she read several more pages into the index, and found that the serial title is actually Морской сборник, and we have a decent run of it!
I brought in a separate bib record for the index (it has a separate publisher than the serial title), but classed it nearby:
V5 .M8 Index 1970/1979