The missing top black thingy (green arrow) is a track end. Meant to eliminate sharp edges at the end of a vertical or horizontal track
image.fisheriessupply.com/c_lpad,dpr_2.0,w_300,ar_1:1,d_imageComingSoon-tiff/f_auto,q_auto/v1/static-images/216048-ronstan-ron-rc72581-t-track-end-ppm-tif The missing one should be replaced to protect the sail.
My guess is that the stainless track has been added to repair some damage to the mast track which allowed the slugs or bolt rope to slip out. However, it has widened or never really fixed the problem.
As Ramona has suggested the sail leach is fed into the bottom of this track and hauled up.
Coming back to the original question:
I don't have anything that would prevent the sail from pulling out of the track if under a lot of stress.
Do you mean pull out at the gap where the green arrow is?
Pull out through the gap in the stainless track?
Pull out at the gap where the red arrow is?
The gap in the stainless track does look wider the than the actual mast track.
I little more information is needed.
Gary