1977 GE 6RS20SP9B9 - what the heck is this component?
I think it is a thyrector, a transient voltage suppresor.
Datasheet : http://docdro.id/eVE6OB5
why (noise)
It is a selenium rectifier.
Google search of the part number points to this.
http://www.parttarget.com/6130-00-136-1416_6130001361416_KZ9DTM.html/-1642D163-C569-435E-881F-72C91B685EF9
MRC Parameter Characteristics
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AARA Terminal Quantity 2
AARB Terminal Type Wire lead
ABHP Overall Length 1.600 inches nominal
ADAV Overall Diameter 0.575 inches nominal
AJUR Circuit Connection Style Designator Half-wave 1 phase
ANPZ Inclosure Feature Plastic case
ASCT RMS Input Maximum Voltage Rating 270.0 volts
ASCZ Stack Quantity 1
ASDS Grounded Terminal Not included
MATT Material Selenium