the m-39 was a gun smith introduced to compete, for an air force contract, with the colt commander. the m-59 (blued, alloy frame) was the double stack version when they went after LE sales. it evolved through the 659 to the 5906.
my favorite feature is it's fast re-set, my least favorite is the weight and the take-down. i've seen a performance center 5905 (blued all steel, sperical bushing) that would hold it's own with a sig 210