1978 124. I found this part lying in the tub when I removed the windshield wiper cowl cover. Anyone know what it is?

Looks like a mat / shield of some form?

It is probably to cover the wiper motor or air cabin intake from direct water falling on and through the windscreen/wiper valence (cowl) cover.

Upon further inspection, you may be right. The lines on it look as if it were in direct contact with one of the cowl cover grills.