Okay, here we go. A bicycle chain bit has a mouthpiece that looks like a bicycle drive chain.
It's one of the bits most likely to be cited as "abusive" - a bike chain bit is one of the harshest bits in existence. Some people find that it works very well on a horse that tends to lean with a skilled rider. They're also called "mule" bits (although I personally don't know that a trained mule needs a different bit from a trained horse).
I personally would not use one, especially an older one - they can be sharp.