A connector system is provided that is especially useful for SCEM (small computer expandability module) systems wherein small circuit boards or "tiles" can be stacked at different positions on a small mother board, wherein each connector includes numerous very small matable contacts that must carry high frequency signals. The contacts of first and second matable connectors each have forward portions (80, FIG. 6) comprising an elongated beam (132) having a straight rear part (140) extending parallel to the mating direction (114) and having a forward part (144) with a sidewardly-projecting protuberance (146). When the connectors are mated, the protuberance of each contact engages the straight rear part of the other contact. Each connector housing includes an insulator with an upstanding support wall (91) having a plurality of grooves (122) spaced along a row of contacts, with each groove surrounding the axis (142) of each contact on three sides, except for the contact protuberance. When the connectors are mated, each support wall is inserted into a slot lying between a pair of support walls of the other connector. Where connectors are required at opposite faces of a circuit board, each contact has a mount portion lying in a plated-through hole of the circuit board, and has substantially identical opposite end portions that each include a beam with a protuberance.