[Milsurplus] Signal Corps Order Number question

[WA5CAB at cs.com]

Group,

I'm looking at a photo of a GN-58-A-GY.  Order number is "U.S. 
DA-91-516-EUC-539".  Does anyone have any idea what year this might have been issued?  
Generator has another plate on it indicating it was procured in West Berlin by 
Signal Corps United States Army.

Robert Downs - Houston
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