During the Vietnam Era Colt made prototype monolithic polymer receiver M16s. These photos are from an online auction where one came up for sale. Plastics and materials technology just wasn’t advanced enough back then for Colt to make this work.
Fortunately plastics technology advanced and the monolithic polymer receiver concept advanced with the CAV-15 In 2000 and then the CAV-15 MKII In 2003. The design team at Cav Arms in 2002-2003 never saw one of these Colt units until after the CAV-15 MKII was in production. So many of the general design concepts and reinforcements are similar. Separated by decades; people that knew the strengths of plastic reached similar design conclusions.
In 2020 the monolithic polymer receiver concept takes another step forward with the KE Arms produced polymer receiver.