Cover panel for screen legs arrowed, simply pop-riveted into position.
What lies beneath (John Holland):
Packing piece AHH6413. Two types of packing piece seem to be available, this from Moss and others:
And this from some other suppliers, with shims AHH6414. Note the raised lettering and plinth on this type that needs to be ground off before fitting. The irony is that the other side seems to have a boss, at one end at least. If the lettering were on a side that had two raised bosses it would be fine. Also note the lack of central hole on the packing piece, which is usually used to secure it and the shims to the body to hold them in the right place to get the bolts through. The bolts go through the body panel, then shims (qty. to suit), then the packing piece, into the screen leg. (Terry Cooper).
Both types of spacer seem to be 'handed' in that one end of each on one side has a larger chamfer than elsewhere - lower left on the Moss item, upper right on the other, ditto the shims albeit less pronounced. It may be that the large chamfer has to go uppermost on the top bolt to clear bodywork.