Context : The Ordnance QF 20 pounder (simply known as 20 pounder or 20-pr) was a British 84 mm (3.307 inch) tank gun introduced in 1948 and used in the Centurion tank, Charioteer tank, and Caernarvon tank. 
Question : What year did Ordnance QF 20 pounder come into use?
1948