Turns out the tank still had a manufacturer's label on it (in a location I originally couldn't get to). Armed with a phone number and model number, I found out the original Utah company was no longer in business, but the person who answered the phone referred me to another welding company nearby, some of whose employees had worked for the OEM and who remembered that, indeed, there was a single baffle, separating the roughly rectangular tank into two roughly square sections. Now I know where not to cut.