The Government's website regarding tax exemption for classic vehicles states that the vehicle build date is the qualifying factor, so vehicles built before January 1st 1984 become tax exempt from 1st April 2024, so we would expect your car to be eligible for exemption in April 2025. This error may have occurred due to the information shown on the V5/C, which only shows the date of first registration rather than the date of build. We would suggest making another application to the DVLA and enclose any evidence you have for the date of manufacturer.