During the winter of 1939-40, development of an upgraded version of the M.S.406 was begun as the M.S.410. The programme was based on the use of existing M.S.406 airframes which were to be fitted with a fixed radiator bath to overcome problems presented by the standard semi-retractable radiator and a revised wing permitting installation of two belt-fed 7.5mm guns rather than one drum-fed weapon. It was also proposed to fit ejector exhausts. The radiator bath and modified wing were flight tested during January and February 1940, 500 pairs of two-gun wings being ordered, and the ejector exhausts were tested in April 1940, these boosting max speed to 509km/h at 4000m. The events of May 1940 interrupted the programme when only five M.S.410s had been completed. At the time, a further dozen conversions were virtually complete and some 150 sets of modified wings had been produced. After the Armistice, a modification centre was established under the auspices of the German authorities, repairable M.S.406s being sent to this centre for conversion to M.S.410 standard. In the event, 74 aircraft were fitted with the new wings, but some were completed as hybrids in that they retained the semi-retractable radiator, and none was fitted with the ejector exhausts. Eleven M.S.410s were delivered to Finland and otherswere included among the Morane-Saulnier fighters supplied to the Croat Air Force.