"The opening to the attic is a small port, 20 inches square, but the only thing up there is a very large wooden box that is bigger than the opening; 69" long, 29" wide, and 20" deep. It seems to be lead lined, and has two long bolts attached on the outside short end. There doesn't seem to be a lid for it but the wood throughout is quite thick, so is likely very heavy."
I dropped by today, got a tour of the house, and then climbed the ladder up into the very short attic space, to try and photograph the very coffin-like mystery box. It turned out not to be lead-lined, but it did have a remnant strip of lead or zinc along one edge, so perhaps it was lined at one point. At the bottom of the box were a number of holes, which look like they were covered with some sort of flange or gasket, possibly to fit a pipe. Due to the confined space and angle, it was hard to get good photos, but this will give you an idea of the box:
My theory was that it was part of a cistern or water basin, possibly to power a gravity-fed flush toilet or shower. The use of the reinforcing rods might support this theory, if the box was meant to hold a heavy quantity of water.
I came back to the office and did some searching, and while I can't say exactly where the box came from, it is very similar to two boxes I found in old catalogues, digitized as part of the rather fascinating Building Technology Heritage Library (BTHL), which is primarily a collection of American and Canadian, pre-1964 architectural trade catalogs, house plan books and technical building guides. If you are a buildings nerd, don't go there unless you have some time to kill!
The first example is from the circa 1912 Wood Tanks catalogue of the New England Tank & Tower Co, made of cypress. Note the rods:
The second is a slightly later example, from the 1937 "Wood Tanks Catalog No. 37" by the National Tank and Pipe Co out of Portland, Oregon:
Mystery solved (with no corpse to dispose of).