
From your photo I can see an after shroud chainplate that runs athwart ship and is probably bolted to a bulkhead below.
Also (vaguely) a forward shroud which is probably rigged the same way to a forward bulkhead..

I have added a drawing of what you have described on the left and what I imagine on the right.

Very often these through deck chainplates have a horizontal plate welded to them which is either above deck (my case) or below deck. These plates give the sealant a surface to bed to and seal the deck from chainplate leaks. I think you will need to go below to measure and photograph your set up.
My question is why are you thinking of replacing the chainplates on such a new boat?
gary