Many thanks for the replies.
This is the response that I got from Walter
These are custom bolts, hardened with anti-vibration patch; they can be ordered by calling 201-656-5654. To prevent the bolts from loosening up, please check your V-Drive to prop shaft alignment as misalignment will cause "pushing and pulling" or the coupling bolts. Since a max prop puts more stress on the coupling bolts when it snaps open, you may want to also use Loctite #242 threadlocker on the threads of these coupling bolts.
THE WALTER MACHINE COMPANY, INC.