There are four large attachement points taking tha loads from the dual aft stays and backstays of the main mast and they are also taking full load of the mizzen mast.
Each attachement point are built up around a large beam going down in the hull. Inbetween this beam and the stay turnbuckles there is a support.
These supports where attached to the beams with a threaded rod M12. At seatrial I replaced these threaded rods temporarly with12 mm rig bolts.
During the maintenance at Åland when I repaired the bowsprit I had Christian to manufacture four 16 mm rods. No threads no nothing, just a rod. At each end there was a 3 mm hole for a lock pin.
Here is a picture of the M12 nut
Below is the final construction.
Bear in mind, each thread of a bolt is a source for cracking, never use bolts or screws taking a shearing load.