Okay, here is my final summary of this topic.
I heard from a number of folks, several of whom I consider to be Healey "authorities." There was a split decision as to whether the front securing fastener on the aluminum moulding is a #10 machine screw with nut, or a #10 sheet metal screw. My guess is that originally the car had sheet metal screws in this location.
However, in my case, and I suspect in many others, the hole in the shroud for the sheet metal screw was enlarged over the years to the point that a #10 screw will no longer tighten down. Going to a #12 to solve the problem results in a screw head that is much too large for the moulding hole and it would not match the other machine screws in the moulding.
I love this the Healey fraternity because people are so willing to help. Jack Brashear came up with what I consider to be the best solution to this issue. Getting a regular nut on the back of the securing screw is virtually impossible. Good luck, too, getting any kind of wrench on the nut to hold it while tightening the screw!
Jack's suggestion was to us a Kep's nut (it has a star lock washer attached to it) to secure a #10 x 1/2" or 5/8" machine screw. See the photos (click on the thumnail for larger image) at the following web site:
https://homepage.mac.com/linwoodrose/PhotoAlbum60.html
As Jack suggested, I stuck the Keps nut on the end of a hacksaw blade with some 3M rope caulk, being careful not to get the caulk in the threads of the nut. I used a 5/16" nut on the back side of the blade to act as a spacer and slid the blade with nuts behind the moulding and shroud lip. You can visually line up the holes and insert the screw, turning carefully until it engages. You might need to hold a flat blade screw driver against the nut until it gets tight enough for the teeth on the washer to grab hold. The hack saw blade was the key - nothing much thicker than the blade will work in the limited space available. That is why I think self tapping screws were used originally as in the door moulding trim, because this labor intensive process would just be too slow for the assembly line!
A very minor point in a total restoration process, but a potentially aggravating one when you get down to the final assembly stages.
Cheers,
Lin
1960 BT7 in restoration
1959 Bugeye