Just bought some M5 x 20, too small, I had some 5/16 bolts sitting around, way too big, same with 1/4 by a smiggin. Took out a caliper and it's 6mm. Just happened to have 2 M6 x 16 in something else, I tried those and they fit perfectly.
So the correct size would be in Stainless Steel:
M6x16...