Hart Convertible Gondolas were dual-purpose cars – designed to act as a regular gondola most days, but could then be converted to a drop-bottom gon for things like ballast and ore. They’re nicely explained in this blog post.
According to Ron Simpson’s notes, the CRNW had nine of these, initially numbered 381-389 and later renumbered 3810-3840 through some unknown mapping. The CRNW versions are noted as having a wooden deck, 40 ton capacity, and 8 truss rods. Ron’s roster also notes they were primarily used for hauling unsacked high grade ore.
I believe these may have come from the Michael Heney’s Copper River Railway based on this old glass negative I have. In it, two cars that look very much like Hart gons are positioned under the Kennecott Mill ore chutes, and both are clearly marked “C.R.Ry.Co.” on the end. The rest of this photo is of an unfinished mill building, so this is early early in the CRNW’s life and having CRR cars around is still plausible.