SKF or Svenska Kullagerfabriken AB is a Swedish manufacturer of ball bearings. They have been in business since 1907 and today are the world's largest maker of bearings. They are famous in the yo-yo world for producing the first yo-yos with a ball bearing axle in 1984, as a promotion for their products. They made both plastic and wooden yo-yos. (They did not, however, make a metal yo-yo; the first metal yo-yo with a ball bearing axle was the Silver Bullet 2.)
The SKF yo-yos today are rare and collectible. By modern standards they are unusual, with a very large bearing, narrow gap, and wheel shape. The halves are riveted in place and do not come apart.