Find location #6!
You solved puzzle 5, well done! 1,5K was the temperature Kamerlingh Onnes managed to reach in his laboratory, at that time the coldest point on Earth. It was a cooled state of liquid helium, another of his discoveries. The instruments he used to do this can still be admired at Museum Boerhaave.
As a reward for solving the puzzle, you get a symbol for the end solution. Remember it well!
And here is a photo clue for the next location of the treasure hunt, puzzle #6: