The slab is cut out for the tub drain, about 20" x 18". I reckon that hole needs some kind of seal after the plumbing work is done these days, but apparently in 1953 they left these holes open to the ground. What is the best way to handle sealing this up?
you dont seal it up. its left open so that if the trap needs to be worked on, you dont have to bust up concrete to get to it. that being said however, 20X18 seems rather a large section. If you still want to seal it up, I'd box out the piping then just use a bag of concrete mix.