So in theory it could have come out of a HK GTS Monaro.
Without seeing the engine number it could have come out of almost any HK-HG. For Australian sale 186S was optional in HK on any model except Brougham and GTS327 and in all HT-HG 6cyl models.
FYI, apart from external differences there is no major differences between 186S and 186 that would actually matter that much today. Yes originally they used on old pre 1967 forged crankshaft, stronger valve springs and better main bearings but most of that apart from the crankshaft would probably be gone long ago. Some 186S had a different camshaft and heavier exhaust valves to their respective 186 but a lot of normal 186 used the same valves and/or camshaft as 186S.