Optional brass "washers" lock onto the socket or terminal tail during the manufacturing process and contact the solder paste screened onto annular rings on the pc-board. When reflowed, the solder wets the lead and plated through-hole, leaving the balance of the tail solder-free for reliable mating with another socket.
Samtec Inc., Box 1147, New Albany, IN 47151, FAX (812) 948-5047.