Thanks for all the replies folks. There is no marking on the head as it's not a bolt. It's a threaded stud and a nut.
Somebody has since explained to me why they use studs not bolts though
"Because a bolt moves with respect to the thread as you apply torque increasing risk of tearing metal.
Stud, threads are stationary, you apply torque to nut so apply direct tensile force to metal of threads rather than combined tensile/surface traction (or frictional shear or whatever you want to call it)"
That makes sense to me so I'm going to stick with the OEM studs and order some from a Yamaha dealer.
Thanks again everybody.
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