Yodon, here is my method for ensuring that I have a complete DMR without needing to have a formally document-controlled index. I admit that my approach is unusual, but it works for me.
My document control SOP only allows controlled documents to go to "Full Release" - Rev A in my case, if all supporting lower level documents are also at Rev A or higher. Therefore, if my finished device specification (aka design output specification) is at Rev A+, then based on following that SOP, all lower level documents must be at Rev A+.
Typical audit scenario would be as follows.
AUDITOR: Show me your DMR index.
ME: I can show you our DMR but we do not require an index to be maintained.
AUDITOR: Ok, show me your DMR then.
ME: Ok, the DMR for what finished device? (REF/Catalog Number)
AUDITOR: REF 1234
ME: Ok, here is the top-level DMR document for DMR REF 1234.
AUDITOR: The DMR is just that document?
ME: No, the DMR is a compilation of documents containing the procedures and specifications for a finished device. If you want to see the entire DMR for REF 1234, you need to keep asking me for documents. You will notice in the top-level DMR document I just gave you, there are references to other lower-level parts and associated documents. You can review each of those documents, and those documents, in turn, may reference even lower-level documents.
AUDITOR: Ok then, let's keep going. Show me all of the documents referenced in the top-level DMR document.
ME: Ok. And as I show you each of those documents, you can request lower level referenced documents if you wish.
AUDITOR: Great! And by the way, this is pretty strange since most companies maintain a DMR Index, but I see now that there is no specific requirement to do so.
ME: It is a pain in the arse to maintain a "living" DMR Index document.