These are model kit upgrade parts to represent the underside umbilical attachment structures that connect the space shuttle to the external tank. They are designed for the Monogram 1/72 space shuttle + external tank and solid rocket boosters model. They include the umbilical ports along with the open access doors.
A template for cutting the square holes in the hull is here:
https://www.dropbox.com/s/f0cyzb3rk4ow6oj/umbilical%20template%202.pdf?dl=0