NimNW,
Use of 'Borneo' suggests Indonesian Air Force. They had Mustangs at the time of the Confrontation. There are some interesting tales (Jackonicko knows more on this) that one of these Mustangs took part in events that undermine the 'the RAF has only shot down RAF aircraft' line somewhat...
As an aside one of the BoBMF Spitfires (I think it joined the Flight later, anyway) was pressed into service to train Lightning mates in the arts of shooting down prop-driven fighters so that the Indonesian Mustangs could be engaged if needs must.