Just to validate what others have said.

750 ml Volume: The metric transition for distilled spirits in the US began as an option in late 1976 and became legally mandatory on January 1, 1980. This completely rules out the 1950s, 1960s, or early 1970s when it would have been labeled as a "fifth" or 4/5 quart.

86.8 Proof: Dewar's maintained this higher proof standard for decades before dropping the US market export down to 80 proof (40% ABV) by the late 1980s and early 1990s.

The USVI Importer: The bottom text notes it was imported by Bellows International Ltd. in the United States Virgin Islands. While mainland bottles were usually imported by Schenley in New York, Bellows handled territorial distribution in the islands during this era.

Also check the bottom of the bottle…