Monitoring Station Designator
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Monitoring Station Designators were used by the United States Army Signals Intelligence Service to designate a specific signal intelligence monitoring station in order to allow analysts to understand the source and type of SIGINT they were analyzing. These were used as shorthand rather than writing or typing out the PLA (Plain Language Address), which consisted of the formal unit name and location of the unit who performed the signals collection.[1][2]
These designators were used from the 1930s through the end of World War II. They were replaced after the end of the war with SIGADs (SIGINT Activity Designators), which are similar in format and application.
Two facilities, Vint Hill and Two Rock Ranch, were operated by the NSA until the 1970s.[3]