It appears to be another Clayton/US Stove furnace with another name (Johnson).. I see it uses the same exact shaker grates that my Clayton uses. Its important that the grates function properly, otherwise he won't have a coal fire burning very long. Coal produces a substantial amount of ash, without a way to shake it out of the coal bed it will quickly (over the course of a few days at best) choke the fire.
Its very possible the prior owner warped the grates and/or grate frame. I saw a grate that a fellow co worker warped so badly it was destroyed. It had sagged into a "U" shape due to continuous over firing and neglect to keep the ash pan cleaned out.