See the small hole at the bottom? I have no idea what that was for. The two holes above it were obviously the hot/cold lines. The two larger holes below were drain lines.
MYSTERY QUESTION #1: But why two drain lines? Was one for the 1894 sink, and the other for the 1929 sink?
MYSTERY QUESTION #2: Why are the drain holes NOT centered under the assumed 1894 vanity or 1929 sink? This is REALLY odd. (NOTE: The sinks never drained through the floor. They drained into the wall, and then to the basement.)
MYSTERY QUESTION #3: In the oval, top left, are more holes. What are these for? Ditto with the holes over to the right. I assume these were for a soap dish and glass holder attached to the white quartz? But the hole patterns are so odd.
MYSTERY QUESTION #4: There should be an outline where the marble vanity kissed the white quartz. Like a thin horizontal line about 32-inches long. But I discern no such clue. When the vanity was removed, was the quartz polished clean?
There is no longer any evidence in the basement ceiling to help with answering the question of how the bathroom was originally laid out. No, the only extent clues are encoded in the white quartz slab.
I feel confident that the bathroom was, indeed, originally a half bath. The vanity was to the left, and held up by brackets. A high-tank toilet was to the right.
Did each fixture rest upon marble slabs inset into the tile floor? I have no evidence to support this. But suspect it.