--- Marvin Johnston <marvin(a)rain.org> wrote: The R-series are for the Straight-8 and the -8/S, as well as some of the peripherals of the day (PT08, DF32, etc.) S-series are, IIRC, performance-selected R-series modules (same PCB, qualified semiconductors). There were also B-series (blue-handled), W-series (white-handled) and a few others in common use during the mid-1960s. As for the M-series, you are correct - they were the primary constituents of the PDP-8/L and -8/i, as well as a few other devices of the late 1960s. By the early 1970s, individual M-series modules with specialized components (DEC bus drivers/receivers) were common as parts of Unibus peripherals. The CPUs had moved to Quad-height cards (OMNIBUS and Unibus). I'm always on the lookout for M-series modules to keep my PDP-8/L, PDP-8/i and RK-11C working. Respond to me offline with what handle numbers you have and let's talk. -ethan