hi Stephen
it does not suprise me much that this should turn up in your neck of the woods
many diferent kinds of mortars and the like turnd up there
i was lucky in that in my service i didnt go to northan irland
but i did much training for going there
we saw many mkany diferenttypes of things in training
and i would believe the soldiers who went there would have seen many more
the sight you show is most defaenately a morter sight
but ,which and from where ,, who knows ,, some one with better knowalage than me !!
im sure some one here can identify it