Clive Elliott GW4MBS
5840-99-112-0319 is not a UN number but a NSN (NATO Stock Number)
The NSC (NATO Supply Classification) 5840 indicates it is for Radar Equipment except Airborne.
The NSN detail reveal although British (99) it was also used by The Netherlands & Denmark
MESL part no 16S1131
No other details I'm afraid but the manufacturer's numbers should help track down some details.
Clive GW4MBS (ex-G8ADP)
Pottering on 6m - 3cm in a valley in IO71XW where any QSO is a triumph of optimism over geography!