Back in 2014 I drove the entirety of Lassen Park Highway through Lassen Volcanic National Park. Lassen Park Highway is an approximately 30 mile National Park Highway which connects the northern segment of CA 89 near Lake Manzanita to the south segment near Morgan Summit. Lassen Park Highway was constructed from 1924 through 1931 as a scenic highway which accesses the flanks of Lassen Peak. While Lassen Park Highway isn't maintained by Caltrans it is the implied route of CA 89 through Lassen Volcanic National Park. CA 89 was once even signed in Lassen Volcanic National Park by the California State Automobile Association.
https://www.gribblenation.org/2020/05/lassen-park-highway.html
I made a return trip to Lassen Volcanic National Park over Labor Day Weekend via the south entrance. That being the case I updated the existing Lassen Park Highway article to include a northbound drive in addition to the previously post southbound. There was some additional tidbits added regarding the history of Lassen Volcanic National Park, some additional trails, and some more information regarding the signage continuity with CA 89/Lassen Park Highway. Notably the Visitor Center has Lassen Park Highway presently signed as "Lassen National Park Highway."
https://www.gribblenation.org/2020/05/lassen-park-highway.html