These resulted from a bug last year with one of the vet rewards. It was reported immediately, including here on statics on more than one occasion. It was finally fixed about a month ago. I do not know how many pieces were made in that 8ish month interim, but I myself made two houses using these. They are EXTREMELY challenging to build with, but the results are nice. My house on Legends just outside the east gate of Luna is an example. I also have a house on Siege made in this way, and I believe last year I passed a house on Atlantic built like this, but I'm not there much at all so can't give info.
My assumption has always been that this is a future crafting option still being worked out that accidentally had its item codes mixed up on the vet rewards options after a release. Until that time the pieces are probably rare, as they cost A FORTUNE in special woods to make. A basic piece would consume 50 to 60 pieces of frostwood, bloodwood, etc. So you can extrapolate that to the cost to make enough pieces for an entire house.