This is the same sprocket that guides #25 chain and drive sprocket. The sprocket has 20 teeth with a .25" pitch and contains an R6 ball bearing with a 3/8" ID (inside diameter).
Basic information on sprockets and roller chain. This video explains the uses of drive sprockets and idler sprocket and goes into some details of the geometry of the sprockets, how they work and various configuration that can be used with sprockets and roller chain. Information on calculating the steps per inch is also explained.