The manual focus Nikon 45mm f/2.8P is a pancake-style lens based on a Tessar design of 4 elements in 3 groups— extremely simple, yet in truth quite well corrected for such a small number of elements.
With lens hood and lens caps, the Nikon 45mm f/2.8P weighs only 156g, making it perhaps the lightest lens one can find for a Nikon DSLR. It is very compact as well. It thus has appeal for any high-res DSLR shooter looking to travel light, at least if the images to be made are in the f/5.6 - f/11 range.
The Nikon 45mm f/2.8P is discontinued but can be found on the used market.
A good alternative would be the Voigtlander 40mm f/2.8 SL Ultron II Color-Skopar, which remains in production and is about $449. In Canon mount, I’d suggest the Canon 40mm f/2.8 STM, a very nice lens, and a steal at about $149.