First we collect a few pictures from prior pages to identify this part, and where it will be installed. The MGA Coupe door window frame includes the articulated quarter light assembly (vent window). At the foremost point of the frame there is a triangular trim cover called Blending Piece (front) that finishes the forward point. Without this pointed cover the frame comes together at this point as two straight ends held together with an angle bracket and screws (similar to the lower corner bracket for the roadster windscreen). When assembled this pointed cover also provides the nest for the quarter light rubber seal in that lower front corner.


The Blending Piece needs to be spread open slightly at the bottom to allow it to slip over the frame, and then will be pinched back together when installing the small retaining screws in the sides. The screws are #4-48-UNF raised countersunk slotted head, chrome plated. BMC factory part number for the Blending Piece (front) is AFH4869, two required of the same part number for left and right doors.



Photos and notes show a small hole in the outboard side of the Blending Piece, implying handed parts (not indicated by the single part number). That last hole takes the screw that attaches the door trim strip at its front end, and the hole may have been drilled after the Blending Piece was installed, and maybe after the window frame was installed in the door.
Similarly we have the Blending Piece (rear) shown below. This requires a U-nut (Tinnerman fastener) to take the rear fixing for the door trim strip. It is 0.313 wide, 0.483 long and takes a 0.138 diameter No 6 screw. The first picture shows the door with its trim strip removed to show the U-nut in position. Not visible in the picture is the fixing of the pyramid to the glass channel which is just to the fore of the U-nut, behind the up-stand of the door skin. The fixing is a tapped hole in the glass channel which I believe to be UNF 6-40 to take a very short 6-40-UNF countersunk screw.

On 8/7/2022, David Adams in the UK wrote:
"For my car, since I could not buy the 4 and 6 series screws, I used BA equivalents which are easily available in UK (being much beloved of model makers). I also used polished countersunk raised head screws in stainless since they look neater in this application. -- Regarding length, I would recommend anyone rebuilding Coupe doors to buy over length screws and cut them down to suit the location.