No predictions loaded for 1991. This site serves NOAA predictions from its own store, and this period has not been fetched for Mona Island yet. Nothing is estimated or interpolated in its place. NOAA's own page for the station has the official table: tidesandcurrents.noaa.gov.
tidealmanac.site/us/pr/mona-island-9759938/1991
Mona Island, PR tide table 1991 (printable) · Tide Almanac