The R and S series FLIP-CHIP boards were used in the original PDP-8 introduced in 1965 and peripherals. The R series was also sold as building blocks for custom hardware. They replaced the original product of Digital Equipment Corporation (DEC) which was the System Modules. The system modules were used to make the PDP-5 which was the predecessor of the PDP-8. The R series boards were replaced by the M series logic using standard TTL logic IC's when IC's became common and affordable. The M series board were first listed in the 1968 logic handbook and used in the PDP-8/I introduced in 1968. The S series boards are identical to the R series boards except lower resistance pulldown on output. This gave them higher speed but reduced fanout capability. Some also had coupling capacitor and other resistor value changes. More R/S series information. The R series logic is explained in this document and the earlier logic handbooks available here.
The R and S series boards have Red handles.
R Series Features:
R series board pictures