From the Service Parts List:
De-luxe Seats (Optional Extra), Seat Assembly - bucket - R.H. (or L.H.)
"If this seat is requested after car delivery, 1 off floorboard - R.H. AHH5748 (or L.H. AHH5740), 8 off screws CMZ0420 are to be supplied also".

Change of floor parts:
  Standard parts:
AHH5205  Floorboard - R.H. 
AHH5206  Floorboard - L.H.
CPZ0416  Screw - fixing floorboard - (8 req'd)
  For C/D seats:
AHH5748  Floorboard - R.H. 
AHH5749  Floorboard - L.H.
CMZ0420  Screw - fixing floorboard - (8 req'd)
    The CMZ0420 screws are 1/4 inch longer and not pointed.

The SPL is misleading when it calls this "Screw - fixing floorboard". These screws are used to "fix" (attach) the seat lower tubular frame to the upper moving slide rails, 8 screws per seat, 16 screws for 2 seats.

As of this writing, I do not know the difference in the floorboards. The adjustable slide rails are not changed. Center distance between the slides is unchanged at 12 inches. Due to shape of the bottom frame mounting tubes, the slide rails may need to be installed farther aft for the C/D seats (but so far speculation).

Addendum June 7, 2023: Okay, we have the answer. For Competition Deluxe Seats, the mounting holes in the floorboards are 7/8-inch farther forward. Not a big change, but with thicker seat back rest this allows for the seats to mount more forward. If it was my car, back in the day, I might just drill more holes in the original floor and not bother changing out the floorboards.

Change of seat colors:
  Fin. (C) 2192 (T/C):
AFH5668  Green - Green
AFH5670  Grey - Grey
AFH5676  Black - Green
AFH6560  Black - Blue
  Com. (C) 2193 (T/C):
AFH6774  Beige - Beige
AFH6776  Black - Blue
  (Different part number for different shade of blue piping?)

The C/D Seat color change point for the Twin Cam coincides with the change from 1500 to 1600 body style. Oddly, the C/D Seats are not listed in the 1500 SPL. This implies that they were optionally available on the early Twin Cam cars, but were not offered as an option on the standard 1500 cars. They are listed in the 1600 SPL. With the note about the different floorboards being required for C/D Seats being installed after delivery, it is clear that the C/D seats could be retrofit in 1500 type cars.