sentences:  Their efforts to improve the design resulted in October 1939 with the SP-200 series, an 18-tube, single conversion superhet receiver.The SP-200 series Super-Pro receivers were manufactured through 1945, with thousands delivered to the military during World War II; they saw wide use by the U.S. Signal Corps as the BC-779.During World War II, government agencies like the FBI used the 200 Series Super-Pro at their listening posts.