Welter transferred to 10./JG 300 on 4 September 1944. 10. "Staffel"/JG 300 was established to counter intrusions by the RAF's fast de Havilland Mosquito twin-engined bombers, flying specially optimized for speed Messerschmitt Bf 109G-6/AS fighters. In September, Welter claimed seven Mosquitos downed, including one by ramming. It is thought that during his service with 1./NJG 10 and 10./JG 300, Welter recorded 12 victories in only 18 missions. On the night of 19 September 1944, according to Hinchliffe, Welter may have shot down Wing Commander Guy Gibson who was leading a 300-bomber attack on Mönchengladbach and Rheydt. Gibson's Mosquito crashed near Steenbergen in the Netherlands. Welter was the only German pilot to have claimed a Mosquito. The claim is unlikely. Welter submitted his victory claim north of Wittenberg in eastern Germany, some hundreds of miles away from the place Gibson's Mosquito was found. Recent research suggests that his victim was Mosquito VI serial "PZ177" of No. 23 Squadron RAF which was shot down at west of Bad Münder, Holzminden. The crew, F/O. K. Eastwood and Navigator F/L. G.G. Rogers were both killed. Welter was awarded the Knight's Cross of the Iron Cross () on 18 October 1944 for 33 victories in just 40 missions.