M_Boats: Battery Location M17

My recently purchased M17 (1977) has what looks like a battery box lid (group 24 size) glassed in the area where the battery would probably go. I am sure this is not a battery box lid as it is reasonably stiff; it just looks like an inverted battery box lid. Questions: Is this the factory setup for the battery? Will a battery box fit into this lid, or is the battery secured directly to this? Also, my boat has a deck fill and vent tube for a water tank, but the water tank is long gone (holes in galley present, but no hose or tank). Where was the factory tank located and how big was it? Thanks for your help. Steve Shenkel M17 Arizona