It's actually "pinless peepers". Originally, the peepers were metal and a pin was inserted all the way through the nares to secure them in place. Now peepers are made of flexible plastic with prongs that fit into each nare hole to secure them, and easier to install and remove.
A word on these pinless peepers. While they can be a great boon in combating bullying, they are not meant to be worn indefinitely. They can cause irritation that can result in calluses that can grow and obstruct the nares, decreasing air flow. I have such a hen, and she labors to get air through her nares.
Due to this unfortunate side effect of pinless peepers, I only let a hen wear them for a couple of months, then I remove them.