Local reference page based on the California State Military Museum page for militarymuseum.org/NavHospCorona.html.
The museum page explains that the Corona naval hospital grew out of the former Lake Norconian resort property, which the Navy acquired in December 1941. The resort complex was converted into a naval hospital almost immediately after Pearl Harbor.
The hospital expanded rapidly during the war. By 1944 it had roughly 100 officers, 184 nurses, and 1,200 corpsmen, and that year alone cared for almost 12,000 patients. Patient load peaked in 1945 at approximately 4,500.
The most relevant detail for Sky Trail is the museum page's explicit preservation of the name Spadra Annex. It states that the administration building, surgery headquarters, special officers' rooms, and the Spadra Annex, all built shortly after the war started, were collectively known as "topside."
That reference places Spadra within the larger Southern California wartime hospital network associated with Corona and helps explain why Spadra appears in both Army and Navy-era medical geography.
The hospital was disestablished on November 1, 1949, reopened in 1951 to care for Korean War casualties, and closed again in 1957. Subsequent Navy scientific and weapons-assessment organizations occupied part of the property, leading to the later Naval Ordnance Laboratory and Naval Surface Warfare Center, Corona Division.