Stanford's holdings for Senate Executive Documents and Reports are covered in a few ways. Many are on fiche, and others are in paper form in the Federal Documents collection. Almost none of them have individual cataloging. This means that you need to use the LexisNexis Congressional Publications database, available on our Databases page, to search and find the SUDOC numbers for individual Executive Reports. Within that database, you can set the Search tab to Report Number, then search by Congress and Report Number, using Executive Reports as a search category. You'll see that the item you need is called the "Convention on Ownership of Cultural Property" and has a number of Y1.92/2: rp.29. Socrates lists no record for that title, but here is an excerpt from the basic Socrates record on Senate Executive Documents and Reports:

Organization: United States. Congress. Senate.
Title: Senate executive documents, reports, etc.
Local note: Use LexisNexis Congressional Publications database to locate individual items and SUDOC numbers. Example: Y1.93/2: rp.39 means 93rd Congress, 2nd Session, Report number 39. This classification covers from the 39th Congress through the 96th Congress. Senate Executive Reports from the 100th Congress on are shelved under: Y1.1/6.
GREEN CALL NUMBER COPY LOCATION
1)Y 1.CONG. SESS.:LETTER OR RP. NO. 1 FED-DOCS

This report is not covered by the microfiche collection, so if we have it, it will be in paper format under the SUDOC number given on the LexisNexis record: Y1.92/2: rp.29. (If you check the shelf in Fed Docs under this number, you'll see that we do, indeed, have the report you need.)

You'll need the print version of the Congressional Record, because LexisNexis and other search engines don't currently cover the Congressional Record before the 90th Congress. The print version is shelved in the basement of Green Library's West Wing, in Federal Documents. The call # will be X1.1: [Congress no.]. These are indexed year by year. Here's the Searchworks listing for the Index to the Congressional Record.