3/4" hub bearing, usually intended for the backside of an extended right-front hub on an oval kart that utilizes the stepped size spindle.
 
Bearing is marked R12 RS
This bearing has an OD of 1.625"
This bearing will work as a replacement Jackshaft Bearing in a Buller Drive jackshaft tube. It has two rubber seals which are removable for cleaning and re-lubrication.