That look like where it goes. My gut feeling says that using the carriage bolts was modern and originally it was just mounted with the wood screws from the bottom only. The carriage bolts were added later instead of screws. It's not just that the carriage bolts are new, also does not look like how it would have been done. If it had carriage bolt when built I think they would have been recessed in a pocket down in the wood not sitting on top.