Three of the four sub-claims in Tapper's statement are fully supported by authoritative sources: (1) the 87-year interval is arithmetically correct (2026 − 1939 = 87); (2) the last royal garden party at the British Ambassador's Residence was in 1939, confirmed by the Library of Congress, the FDR Presidential Library, and the official Royal Family website; and (3) King George VI is correctly identified as King Charles III's grandfather, confirmed by BritRoyals, NBC News, CNN, and the Royal Family's own website. However, one sub-claim contains a material factual error: Tapper's transcript states that the 1939 event was 'hosted by Queen Elizabeth and her father King George VI,' describing King George VI as Queen Elizabeth's father. In fact, King George VI was Queen Elizabeth's husband — the two were the reigning King and Queen consort of Great Britain in 1939, and together they are King Charles III's grandparents. George VI was the father of Queen Elizabeth II (a different person), not the father of Queen Elizabeth (the Queen Mother, née Elizabeth Bowes-Lyon). This is a clear genealogical inversion confirmed by multiple independent sources. Per MOSTLY_TRUE methodology, the core assertion — that 87 years have elapsed since a royal garden party at this location, last held in 1939 — is substantially correct with one identifiable inaccuracy (the family relationship stated) that does not reverse its directional meaning; a reasonable viewer would still agree with the overall historical point Tapper is making. The error is factually meaningful, however, as it misidentifies the spousal relationship between the two principals of the 1939 event, warranting a MOSTLY_TRUE rather than TRUE verdict. No correction was issued by Tapper or CNN.