Quick facts:

• Model HB-6400;
• introduced in 1988 and was the first time that Shimano used the word "Ultegra" in association with their 600 series groupsets;
• weighs 137g;
• uses a cup and cone bearing arrangement with 11 steel balls in each side;
• has a steel thru axle, threaded all the way;
• was available in 28, 32 and 36h versions;