You are bidding on an Altec Model 15356A Audio Line Transformer.  It is in nice clean condition, except that the octal key is broken off.  So care should be taken when placing it in a socket.  It is designed for 600 ohm line to 600 or 150 ohms for microphone, preamp, or phono inputs.  It was typically used in an Altec amplifier.  It measures 1 1/4 inches in diameter, and 1 3/4 inches tall, not counting the pins. It is a high quality transformer that should be great for your amplifier project.  Shipping weight is 9 ounces.  Please see my other auctions for more interesting radio and audio items, and tubes.