This is a great site for sure! While looking at the "eye nuts" on the projects page, I noticed the partial image of the overhead opening hatch in the head. Ron, did you replace the vent with the opening hatch?
I have thought about replacing my 4" vent with an opening hatch, but wasn't sure. My hull number is 354, 1987 Mark I.
Tom : C34's 1988 and later come with a Bomar Nibo 10" (inside measurement - 12" outside) hatch. What you were really looking at from the inside was the trim ring for the screen. Ron