The Steiner family should have defended their rights to the design years ago. Selling the remaining original stock of the pieces in 1966 and not going any further with it would seem to have abrogated whatever "rights" remain.
Who said the Steiner family didn't do anything to "go further with it"? Did anyone bother to ask?
Beyond that, I find it incredible that you would place the onus on the Steiner family to have "defended their rights to the design," rather than on those who would presume to deny those rights exist.
I am thinking you can reproduce any piece since nothing was filed for. But hey I am not a lawyer, just a person who enjoys collecting chess sets that are repros since I will never have the deep pockets for the originals.