Should I be concerned with this?
This is the Tuttle head on my brand new never used 85cm aluminium mast
(no prizes for guessing which very popular brand)
When I was assembling it I found the rear barrel nut was loose so I tapped the plastic Tuttle spacer off and found they'd drilled a 6.5mm hole in that part of the mast which was only 5.5mm thus breaching the sides and severely weakening the structure where the rear 6mm Tuttle screw attaches to the board.
I realise the front screw takes most of the impact, but this has me a bit worried......
Wouldn't want to overtighten the rear screw as it's all very flimsy.

Don't use it. Take back/return it and get a new one. That whole area is high stress, and that doesn't look at all right.

Yep, agreed with Subsonic, don't use it.

Definitely a very important part of the head, rear bolt takes nearly all the load under normal foiling conditions due to the lift coming all from in front of the mast
I would be very disappointed too if I were you but would have to take it back to dealer for a resolution for sure

This is what it looks like with the plastic Tuttle spacer installed, so no one is any the wiser to what's underneath.
Not that it would add much if any strength imho.