The increasing obsolescence of the M.S.406 led the Finnish Air Force to order, on 22 October 1942, the installation of a Klimov M-105P in an M.S.406 airframe. The M-105P engine, derived from the HS 12Y, afforded 1,100hp for take-off, and a substantial quantity of this power plant, together with suitable VTSh-61P propellers, had been captured by the Wehrmacht and was available to the Finns. A 20mm MG 151 cannon was mounted between the cylinder banks, a Bf 109G oil cooler was adopted, an aerodynamically-improved engine cowling was introduced, and, with some local structural strengthening, the prototype conversion was flown on 4 February 1943 as the Mörkö (Ghost) or Mörkö-Moraani. Successful trials resulted in the decision to bring all surviving M.S.406 and M.S.410 fighters to Mörkö standard, but only two more were completed before termination of the Finnish-Soviet conflict. Nevertheless, the conversion programme continued, and by 21 November 1945, the remaining Morane-Saulnier fighters had been modified, bringing the total number of Mörkös delivered into the Finnish inventory to 41 aircraft. These retained the two or (in the case of the M.S.410 conversion) four wing-mounted 7.5mm machine guns, but shortages of the MG 151 cannon necessitated this engine-mounted weapon being replaced by a 12.7mm Berezina UB machine gun in some aircraft. The Mörkö remained in service until 11 September 1948, when the survivors were placed in storage, being scrapped four years later.