WARNING WILL ROBINSON - WARNING, WARNING
1. The 'squares' are initiation sites for fatigue propagation .... the reason why the British DeHavilland "Comet" jet airplane was such a catastrophic (fatigue) failure. .... fatigue propagated from the 'corners' of the square 'windows', etc. Cut 'squares' are verboten in cyclical stress applications. - a fatigue 'stress riser' that mechanically (grossly) reduces the load bearing ability of the part in 'fatigue service'. If you somehow 'need' those square holes .... beef up the cross section by at least 4 to 5 times (just a wild assed-guess without data books and 'slide-rule').
2. You will probably also see the beginnings of these 'bolt hole' smiley faces (cracks eminating from or from near the 'corners' of the 'squares', in all your other 'chainplates', too ... if not now, then soooon.
This is a 'projected' bearing (saddle) stress enhancement and enhancement of fatigue failure from the 'corners'. It means that the 'point loading' from the bolts TO the chainplates are whats 'transmitting' the stress ---- VERY VERY BAD in design vs. cyclical stress ... and has been so since the crashes of all those Dehavilland Comets in the early 1950s!

Rx:
1. Replace to plates with 'round' holes --- ALL the plates.
2. use 'large' washers between the bolt head and chainplate so you dont 'gall, scratch, etc.' the plate.
3. The bolts should fit snug in the 'holes' ... not much 'slop'. Shoulder bolts or bolts without threads near the 'head' are preferred.
4. MOST IMPORTANT .... use a torque wrench (to approx 80% of max torque value of the bolt) ... and the FRICTION generated by the force of the bolts into the washer and the chainplate below, will hold the 'joint together' and the friction generated (FRICTION JOINT) will transmit the stress over and through a MUCH LARGER surface area .... not depending on the 'strength' of the bolt nor the 'strength' of the chainplate. The bolt strength is there only as a 'backup' in case the torque which causes the appropriate friction forces to 'relax' (in good design).
Of course you have to have a torque wrench, and you must re-torque prior to any important passage. You torque and re-torque with bolt threads 'oil lubricated'.

Take the pledge: Never ever 'hang' a side load from a 'bolt', the bolt is there only to provide the high friction forces between the mating parts.

(Beneteau seems to now impressively have this all worked out to near perfection with their T-bolted chainplate 'rods' - elegantly simple, and all pure/simple stress connection which minimizes fatigue cracking, etc.)