Unpublished error date (768 AH) - Silver tanka of Barbak (1459-1474), mintless type w/error date, Bengal Sultanate, India

Rukn al-dunya wa'l din abu'l muzaffar barbakshah al-sultan bin mahmud al-sultan / al muayyad bi-tayid al-rahman khalifat allah bi'l hujjat wa'l burhan, date below. 26mm, 10.8 grams. No mint name. Dated to 768 AH (sic) - error for 867 AH / 1462 AD. Unpublished with this error date! "The coins of Indian Sultanates" #B-543.