Numerical digit
From India, the thriving trade between Islamic sultans and Africa carried the concept to Cairo. Arabic mathematicians extended the system to include decimal fractions, and Muḥammad ibn Mūsā al-Ḵwārizmī wrote an important work about it in the 9th  century. The modern Arabic numerals were introduced to Europe with the translation of this work in the 12th century in Spain and Leonardo of Pisa's "Liber Abaci" of 1201. In Europe, the complete Indian system with the zero was derived from the Arabs in the 12th century.
De Havilland Mosquito
Until the end of 1942 the RAF always used Roman numerals (I, II, ...) for mark numbers; 1943–1948 was a transition period during which new aircraft entering service were given Arabic numerals (1, 2, ...) for mark numbers, but older aircraft retained their Roman numerals. From 1948 onwards, Arabic numerals were used exclusively.