The Hammarlund SP-600 was produced with many variants mostly from 1951-1955, but later versions were manufactured and available through the 70's. There were approximately 40 versions with various contracts to satisfy specific needs for service. This brief description (with a comprehensive manual for download), accurately describes the "JX-21" model. This particular unit (above pictured) was delivered to the David Sarnoff "RCA Center" in 1957 with a paint scheme that matched other RCA equipment at the time.
From the Air Force Technical Manual- TO 31R2-4-101-2 :
(V1) 1st RF - 5749/6BA6W