PART NO.  SPL.PAGE  DESCRIPTION                                                           
AJA5070   15.N.6    Screw (4 for tail lamp lens 1500/1600)
37H5410   16.N.10   Screw -lens fixing (4) -(tail, road lamps, 1600) -Fin.(C)100351
37H5410   TC.N.8    Screw -lens fixing (4) -(tail, road lamps, Twin Cam)

Substitute part numbers: . . . . . See item 2 at right.
57H5569 (Moss Motors UK)
158-018 (Moss Motors USA)

One thing they all agree on is that it is a dog point set screw (which is why they are all special drawing numbers).

For more information on these screws, see Part Number 37H5410

Rimmner Brothers 57H5569

EnglishParts.com 57H5569

Moss Motors UK 57H5569

Brown and Gammons 57H5569

This part number does not appear in any MGA Service Parts List. It is offered as a functional substitute part.

My best information implies that 1500 cars originally had straight slot pan head screws, while 1600 cars had Phillips slot round head screws, and the substitute part numbers are functional aftermarket parts with different head style. There are other opinions, but so far no one seems to have definitive proof.