How old a boat?
To answer your question, you have to do your own examination of the vessel. You may find the original tank had a problem and was replaced or a previous owner decided they wanted more capacity. That's something that I would make the measurements on and calculate the capacity.
Regarding overboard discharge of the holding tank, it's generally pretty easy to follow the plumbing and know for sure. On my boat, I removed the overboard discharge as it was something I would never use and eliminated the failure points. If the next owner wants it, it's pretty simple to put back as the plugged seacock is still there.
Ted