u/Haunting_Program_298

1962 cabin, NE USA.
This is what I actually found inside used as cement backer for shower tile. It is different than #1 (which I found buried outside in fire pit) in that it has large squares and appears more modern. What do you think?

Found in cabin built in 1962. Very strong cement board that breaks hard. Found in bathroom behind tile. Then I found this panel outside in the fire pit that may have just been dumped there by previous construction. NE USA