I was always told that the stamping on the backing plates should face towards the threaded plate! Something to do with the angle or arch that is engineered into the plate to help grip the metal frame corner.
Now I wonder which is correct. Maybe the hardware mfgrs. will chime in and set all of us straight.
I'm glad this came up because I was never sure which way the writing on the plates should face. I've seen conflicting information. I face the writing up, toward the screw. It just seems logical to be able to see the stamped letters when placing the angle piece in the frame.
The backplates are slightly asymmetrical, so if they are facing the right way their profile will match that of the tapped corner, with no "excess" peeking out from under one end of the tapped plate.
You can apply a dust cover on the back of a metal frame housing such as an element, with a broad flat foot, just as you would a wooden moulding.
A self adhesive linen tape, I prefer Linco's for this because it is not as stiff as the Neschen, for the metal foot like the profile 15's; fill the rabbet, don't leave any space, don't use spring clips, lay the tape along the inside edge of the foot, turn down over the filler board, overlap and trim at the ends.
Actually Alan Sturgess? had a thread on here about a year and a half ago talking about metal hardware and the difference that they had. I am not clear on it, but search his name and his posts. He is extremely smart man with too much knowledge, I miss his responses. A very valuable source that is missing. If you are out there say Hi
I've since started using the Nielsen Quick Corners and really like them. They are quicker (thanks Mar!), and no back plate to dink with, which, BTW, I never saw that it made a difference which way they faced...however they were when I grabbed them is how they went in. Don't have to think about it anymore.
A few years ago, I tried a batch of the Quick Corners, but switched back to the tapped corners and back plates.
Since all of the pressure on the metal rail from the screws was concentrated in one spot, it left a dimple in the slot which often resulted in a bump on the other side. That bothered me a little for some reason.
It seems to me that back plates distribute the pressure more evenly, so I don’t see that distorted dent any more.