Without re-smelting the bars and taking out other impurities or metals, it is hard to know the true percentage of gold content. You can contact metal, mineral or gold assay companies, mining companies, and talk with them for more information. The bars could be gold plated as they were poured, but inside could hold for instance lead, iron, etc. Good luck in your findings.
I would agree with a metal detector find. In this condition, I am going with it being in the ground rather than water, as water can smooth the devices more. It was also not in circulation long after it was Minted in 1946. Us detectorists do put coin finds back in circulation, however, I do not think this is the case. Great find for condition.