This is a vintage thread holder or thread caddy by Scovill, probably from the 60s. It's a rectangle shaped clear plastic box with MCM star on the top. Inside there's 24 spools of sewing thread in assorted colors. Thread lots like this are great for doing visible mending or repairs on vintage clothes, fine embroidery or using for hand sewing.
The box measures 8 3/4" wide X 6 1/2" front to back X 2 1/4" tall.