Jan '21 Update: They Have Been Identified Via DNA; Their Names Not Yet Released to Public -- Who Murdered this Couple? Was the Man "Jock or Jacques," a French Canadian from a Wealthy Family? The Sumter County (South Carolina) "John and Jane Does." Click
"On August 9, 1976, the bodies of an unidentified white male and white female were discovered on a dirt road 1/2 mile from Interstate 95 and Highway 341 in Northeast Sumter County. The road is known as Locklair Road, a frontage road just off the interstate. Both victims were shot multiple times:" NAMUS. There are many clues to their identities, including the initials "JPF" on the man's ring ("J" for "Jacques" or "Jock?"), witnesses who met and saw them before their killings and their good health and professional dental work (likely done abroad for the man). Yet they remain unidentified decades later. BTW, DNA testing determined they were not, contrary to speculation, siblings.
More detail and post-mortem photos of the couple can be found here.
Resources: Missing Persons List + Search Missing Persons by State + Unidentified Remains
NAMUS: Federal government database for missing, unidentified and unclaimed people. The “official source” used by law enforcement. Allows you to track cases. https://www.namus.gov/
Charley Project: Missing persons database with good detail on cases. https://charleyproject.org/
Doe Network: Current missing persons cases and unidentified remains. Includes international cases. http://www.doenetwork.org
Missing Veterans: Detailed profiles of missing veterans. http://www.missingveterans.com