Ok so results are in:
Pool store 2, Tracer pocket tester salt pen - 2450ppm
Pool store 2, AccuBlue - 1792 ppm
Pool store 2, K1766 - 2600ppm
My new K1766 purchased from pool store 2 - 2600ppm
So I guess my K1766 from Amazon is bad (figures). This was an out-of-cycle purchase for me...