The "(XN-1)" part of the nomenclature indicates that this is/was a pre-production or prototype unit. The "/AG" part indicates that it is/was intended for airborne use (the A) in telegraph (teletype) service (the G). The 26.5 VDC further indicates airborne use. It was procured under a Navy contract, according to the contract number on the nomenclature plate. I suspect the unit is of early 1950s vintage. Ed Greeley