I contacted Dr. Danielle Adams, Deputy Director for Marketing and Communications at the Lowell Observatory. She was kind enough to reply, and generously provided the following:
...this is the 24" Alvan Clark & Sons refractor, commissioned by observatory founder Percival Lowell in 1895 and completed in 1896. It was fully restored in 2015, a process that took 18 months, since we restored or replaced everything, down to the screw.
The Clark Refractor, as we typically call it, was first used to study Mars and other planets by Percival Lowell, but it also has a storied history in studying deep-space objects. In 1912-1914, VM Slipher used a spectrograph attached to the Clark Refractor to study the redshift of M31, the Andromeda Galaxy, and in doing so he found the first evidence that the universe was expanding. (Edwin Hubble later used this data and his own research to refine this.)
In the 1960's ('61-'69), the Clark Refractor was used by the USAF's Aeronautical Chart and Information Center to develop highly-detailed maps of the moon, using a combination of photography and professional illustration at the telescope during exquisite moments of seeing. Today, the Clark Refractor is no longer used for research, which allows us to dedicate it fully to public observing for guests who visit the observatory. We also recently started after hours sessions for guests who want to spend more time with the Clark after the observatory has officially closed.
Lowell Observatory's Historian, Kevin Schindler, literally wrote the book(s) on the Clark Refractor. There's a link to his latest book (post-restoration) and a mini-documentary about the process here: https://lowell.edu/history/the-clark-refractor.
In response to my comment above. she continues:
Also, note that in response to the connected question about which wood was used to line observatory copulas, in the case of both the Clark Refractor and the Pluto Astropgraph at Lowell Observatory, local Ponderosa Pine is the sole that was used for the domes. Flagstaff is home to the largest contiguous grove of Ponderosa Pine in the world, a tree that grows tall and straight, making it perfect for building.