I only think I saw these in a Rainbird catalog, some years ago. It was a straight-sided valve box with removable end pieces. You could take two boxes, and pull off two end pieces, and join the remaining pieces together, and get a double-length box. Or triple-length, if you did it with three boxes. Pretty pointless for new installs, when you could just use standard boxes, at a better price (I think - been awhile) but very useful for retrofitting old installs where any number of zone valves might be manifolded together, with no consideration to repairs or manual valve operation. The molds that these were made from must still be around, but who has them, I do not know.