On my k-37 it is m8x20mm. It is also a low head- 5mm high.I'm not sure why the thin head, perhaps for clearance when the blades are tilted- it's the same cap screw for the main blade.
I bought a box of replacements from McMaster Carr. I can send you a couple.
I replace them every couple years when they start to get a little rounded.
Lloyd
Oak Park IL
