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Early in 1988 I had a chance to review briefly some American postal history reference works, and gleaned the following information

Arkansas. In addition to the present Cave City, Cave Creek in Newton County existed from 1855 to 1895, when it changed to Cavecreek. It was discontinued in 1973.

Cave Springs in Benton County began in 1908 and was still in operation as of the date of publication of the reference work.

California. In addition to Cave Dale in Sonoma County previously mentioned in SSC, there was a Sink post office in Los Angeles County from August 17, 1859 through February 14, 1861.

Colorado. Grotto, Jefferson County, was in existence from March 7, 1881 through March 21, 1882.

Kansas. Cave, in Gray County, was in existence from January 19, 1901 through September 30, 1914. Cave Spring, Butler County, existed from June 22, 1874 through July 30, 1878. Cave Springs, Elk County, was from January 29, 1885 through March 31, 1903.

Montana. Cavern (Jefferson County) - served the neighborhood of what is now Lewis and Clark Cavern State Park from March 29, 1909 to January 1, 1936. Cavetown in Meagher County existed only from April 2, 1878 to November 9, 1880.

New York. In addition to Howe Cave/Howes Cave, Grotto was in existence from 1892 to 1902, in Tompkins County.

South Dakota. Wind Cave had its own post office from October 18, 1892 to December 8 of the same year.