I would say British from the Trilux markings, as it is the same as the small arms sight 'SUSAT' (sight unit small arms trilux). For which weapon I can't help you, something reasonably big though I think. Dated 1987 is it?
The NSN is shown here as "TELESCOPE,ELBOW"
Defense and Aerospace Sustainment Partner
"99" - the 5th and 6th digits indicates the UK.
I will have a further dig round later on if no-one has come up with an answer.