Text

Sede Vacante (Latin for "the seat being vacant"), refers to the vacancy of the episcopal see of a particular church in the Canon law of the Roman Catholic Church. More particularly, Sede Vacante refers to the vacancy of the Holy See, which occurs after the death or resignation of a pope. In this case the particular church is the diocese of Rome, and the "vacant seat" is the cathedra of Saint John Lateran. During this period, the Holy See is administered by a regency of the College of Cardinals. 
Benedict XVI ended his eight-year tenure as Pope on 28 February 2013. The Pope became known as Pope Emeritus Benedict, determined he did not have the strength to carry on as the 265th successor to St. Peter as he believed it required. He retired to a life or prayer in the Mater Ecclesiae Monastery on the Vatican Gardens grounds. His abdication was the first since Pope St. Celestine V in 1294. 
On 1 March 2013, four Sede Vacante stamps, designed by artist Daniele Longo, were issued by the Vatican post office. The image on the stamps depicts an angel holding an umbrella and a set of crossed keys, the coat of arms for the Holy See during the Sede Vacante. The stamps were in use during the interregnum between the start of Benedict XVI's retirement and the election of Francis as pope on 13 March 2013. The values corresponded to postal rates within Italy, to Europe, and to the Americas and Oceania. The stamps were issued in sheets of 10. 
The actual number of the stamps printed is unknown. Between 1 March 2013 and 7 March 2013, over 150,000 sets of the Sede Vacante stamps were sold. Sales continued until the 'new' pope was elected on 13 March 2013. 
Reference: <a href='https://www.catholicnewsagency.com/news/26734/vatican-post-office-sells-over-150k-vacant-see-sets'>'Vatican Post Office Sells Over 150k Vacant See Sets' on CatholicNewsAgency.com</a>

Update A Postal Database Record