That's from the 50/60's I believe. Was mainly for export markets like the US and UK.
Unfortunately with it opened it will not hold much value. I would say have a taste yourself (don't worry it won't poison you) and if it is half decent you might be able to sell it to someone who would like to taste it. You wont get huge money though. PotStill of old had a strange near mould type taste so don't let that put you off but if it is sour and tongue sappingly dry or seems diluted it may be well turned (or oxidised and therefore not really upto scratch). Oxidation is where the whiskey evaporates where mainly the alcohol evaporates.
I'd be interested in your findings