Complex sentence: IIc, and this led to a production of a single flight of such designs starting in July 1942, but they had such a low priority the conversions were not complete until the spring of 1943.
Split into simpler sentences: Iic, and this led to a production of a single flight of such designs starting in July 1942. These conversions were given such a low priority that they were not complete until the spring of 1943.