On September 29, 1476 printer Wiliam Caxton name was entered in the account role at Westminster Abbey for having paid a year’s rent in advance for premises there in which he set up his press. The first surviving printing that he completed with a definite date was Indulgentia (A Letter of Indulgence) by John Sant, Abbot of Abingdon, for promoting the war against the Turks. ISTC is00163100 cites only one copy printed on vellum, imperfect, in London at the National Archives, noting "The copy known was issued to Henry and Katherine Langley on 13 Dec. 1476."
"The form is set in William Caxton's types—his second and third. Caxton's first types he had previously used in Bruges. Caxton was employed in Bruges as late as 1475,and probably moved to England in the middle of the following year: his tenancy of a house in Westminster began at Michaelmas 1476. The first of his books dated at Westminster was finished by 18 November 1477. No other printer worked in England until 1478.
"This form, therefore, ranks as the first recorded piece of printing done in England. Its existence in the Public Records was noticed in 1928" (Printing and the Mind of Man. Catalogue of the Exhibitions at the British Museum and at Earls Court, London  no. 2.)
Because it was unknown to historians until 1928, the first Caxton imprint was not included in the Caxton quadricentennial celebration of 1877.