This is a guiding eyepiece used by observers with the Prime Focus Spectrograph of the 200 inch Hale Telescope at the Mt. Palomar Observatory operated by the California Institute of Technology. The eyepiece monitored the reflection of an image off the jaws of the entrance slit of the photographic spectrograph.
The prime focus spectrograph was designed and manufactured at the California Institute of Technology in the late 1940's for installation on the then-new 200 inch Hale Telescope. It was in use almost continually from 1950 through 1973 providing a wealth of data on the redshift, on white dwarf stars, and on quasars. It was donated to NASM by the California Institute of Technology in 1998. It is now on display in the "Explore the Universe Gallery" along with the Prime Focus Spectrograph.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.