That slot is necessary, because the centerboard cap extends higher than the floor boards and stringers for most of its length. The first forward most several inches are flat, but the cap then angles upward as it's goes rearward. This board A goes down either side of the raised portion of the cap.
As Roger said, by pulling the slotted board "A", you can access the entire length of the centerboard and its mounting bolts for repair. But you have to pull the companionway step. On my boat, the entire step and port settee is a single board and a bit PITA to pull out.

And trust me, servicing the centerboard is a lot easier when you have the floor/sole to kneel on, and you aren't trying to kneel between the stringers, etc.
My only suggestion is to add some sort of tab on the aft edges of board A to keep it level with the boards 3S and 3P in the drawing.

Todd T.

On Mon, Mar 25, 2024, 7:25 PM ROGER PIHLAJA <roger_pihlaja at msn.com> wrote:

> Hi Reuben,
>
> My subfloor does not have that slot.  The companionway step covers quite a
> bit of the centerboard cap.  To get at my centerboard cap, I have to remove
> the companionway step.
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
> 1978  Sanford, MI
>
> Sent from my iPhone
>
> > On Mar 25, 2024, at 6:13 PM, Reuben Mezrich <reuben.mezrich at gmail.com>
> wrote:
> >
> > Yes, I also was thinking it’s easy to pull up that panel and so was
> wondering why go to the trouble of putting in the slot. And besides, you
> still have to cover that slot with something to make the floor look nice
> > Reuben Mezrich
> > Cell:410-499-8922
> > Pelican Cove in Sarasota and Back Bay in Boston
> >
> >> On Mar 25, 2024, at 4:26 PM, ROGER PIHLAJA <roger_pihlaja at msn.com>
> wrote:
> >>
> >> Hi Reuben,
> >> I think that slot is to provide access to the centerboard cap without
> removing the cabin sole subfloor.  Theoretically, you should be able to use
> the slot to remove the numerous screws securing the cap to the flange on
> the centerboard trunk.  Removing the cap will give access to the
> centerboard from inside the boat.  In reality, it’s probably easier/faster
> to just pull up your subfloor board labeled “A”.
> >>
> >> Roger Pihlaja
> >> S/V Dynamic Equilibrium
> >> 1978. Sanford, MI
> >>
> >> Sent from my iPhone
> >>
> >>>> On Mar 25, 2024, at 3:36 PM, Reuben Mezrich <reuben.mezrich at gmail.com>
> wrote:
> >>>
> >>> Joe
> >>> My cabin is dry, a bilge pump is now installed and wiring, batteries,
> >>> switches and battery charger have all been replaced. I'm now ready to
> >>> replace the floor (Stan had used cement board covered with thin carpet
> on
> >>> my boat). I intend to copy what you did.
> >>> One question (I expect there will be more), is why is there a slot in
> board
> >>> "A" (the one over the centerboard trunk). What covers that slot in the
> >>> final veneered floor?
> >>> --Reuben
> >>> Reuben Mezrich
> >>> cell: 410-499-8922
> >>> Pelican Cove in Sarasota and Back Bay, Boston
> >>>
> >>>
> >>>>> On Tue, Dec 19, 2023 at 3:44 PM <jpd9668 at gmail.com> wrote:
> >>>>
> >>>> Photos did not attach the first time. Here they are again!
> >>>>
> >>>>
> >>>>
> >>>> From: jpd9668 at gmail.com <jpd9668 at gmail.com>
> >>>> Sent: Tuesday, December 19, 2023 3:27 PM
> >>>> To: 'The Rhodes 22 Email List' <rhodes22-list at rhodes22.org>
> >>>> Subject: RE: Cabin Sole PHOTOS and DRAWINGS
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> These drawings and photos accompany previous post of Cabin Sole
> Project.
> >>>> Sub-floor and Sole drawings in both .pdf and .jpg formats.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> Joe Dempsey
> >>>>
> >>>> Trojan 42 Motor Yacht
> >>>>
> >>>> M/V Voyager
> >>>>
> >>>> Ex-Rhodes 22
> >>>>
> >>>> S/V Respite
> >>>>
> >>>> -------------- next part --------------
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> >>>>
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> >>>>>
> >>>>
>