Answer: Saint Scholastica is the patron saint of cloistered nuns. There are several articles cited below with some additional information on Saint Scholastica.

There is no Vatican City stamp depicting St. Scholastica, although she is depicted on a mosaic which served as a San Marino issue on St. Benedict.

See also:
• Saint Scholastica, Saint of the Day (website), https://www.franciscanmedia.org/saint-of-the-day/saint-scholastica
• James C Hamilton, “St. Benedict: From Subiaco and Monte Cassino to Patron of Europe” : Vatican Notes, Vol. 60, No. 351, pp. 10-18 (2012)
• Old Saints, New Questions: St Scholastica and the Place of Women in the Church, ibenedictines.org (website)
• John Paul Meenan, Editor, Scholastica's Last Day With Benedict, Catholic Insight, February 10, 2021