The AGV MS/ML sizes complicate things a bit further, as well. According to their size chart, MS is listed as 57cm, ML is listed as 58cm. Additionally, there is a jump in shell size between the two (at least there was for the K3 SV I tried on). I measure 57.5cm, right in the middle for most manufacturer's medium size. I first tried on the ML size AGV, and it seemed comfortable. Always wanting to make sure I had a snug, but not uncomfortable fit, I wondered if going down to the MS might work, and you get the smaller shell size to boot. Well, I could barely get that thing on my head. And once I did, I had a mild moment of panic that I would not be able to get it back off without a Dremel tool. My guess is the MS in their (AGV's) line is similar to the M/L example you gave for the Kabuto.