The PN-11 extension tube is a companion accessory to the amazing, popular and very long-lived Micro-Nikkor 105mm f/2.8 Ai-S lens. This lens was introduced in the early ’80s but it’s still in production up to this day. Having said that, the PN-11 is obviously in-production as well. You have to buy the PN-11 separately which doesn’t make sense since it is so vital to the Micro-Nikkor 105mm f/2.8 Ai-S’s performance and it is needed for the lens to achieve a 1:1 (almost) reproductive ratio. Why Nikon sold these separately, I don’t know.
Here are pictures of the PN-11.