R-391, another variant of the R-390, allowed choice of eight different autotuned channels. Three years later, Collins delivered the R-390A to the military. About 54,000 were produced and the set was a military workhorse until the . Like the R-390, it can outperform many modern radios, to the point that it was designated top secret until the late . In 1958, Collins replaced the 75A series with the much smaller 75S series, part of the S/Line. These featured mechanical filters, very accurate frequency readout, and excellent stability. At the request of the US government, Collins designed the 51S-1 general-coverage set, which was essentially (in intended use) a physically smaller replacement for the 51J series. It was not intended as a replacement for the higher-performance R-390A, and unlike the R-390A, it was extensively marketed for commercial use. Collins produced a few high-performance solid-state receivers in the , such as the 651S-1. Like their tube predecessors, these are coveted by collectors today. Transceivers and systems With the introduction of the S/Line in 1958, Collins moved from designing individual products that could be used together, to ones that were designed to integrate and operate together, in various combinations, as a system. They were the first equipment maker to take this approach. Collins was also the first to introduce a compact HF transceiver, the KWM-1, the year before. Together, these two innovations put Collins temporarily ahead of its competition, and set the stage for other manufacturers and the next generation of amateur (and military) HF radio equipment. The 75S-1 receiver and 32S-1 transmitter, comprising the heart of the S/Line, operated separately or together to transceive. The units included crystal band-pass filters and a new compact design that provided stable, highly linear tuning across 200 kHz band segments. The S/Line tuning-dial mechanism was unique when introduced. It used concentric dials and a gear mechanism that provided precise dial resolution, better than 1 kHz. Within a few years, Collins had introduced additional S/Line components, including the 30S-1 kilowatt power amplifier, the 30L-1 desktop power amplifier, and the 62S-1 transverter, which provided coverage of the 6-m (50 MHz) and 2-m (144 MHz) amateur bands. The KWM-2 transceiver replaced the KWM-1 using many of the S/Line's design features and matching its styling. Other accessories included speakers, microphones, and control consoles. Illustrating the uniqueness of their new, smaller units in the market, Collins advertisements in the 1950s and early