Never seen them either. I knew a couple that made several hundred CMU's in their back yard years ago, using a slip form that yielded an 8X8X16 block. On one face they plopped in two field stones that would show on the final product. They had maybe a cpl dozen forms and gathered the stone over time, making the blocks on weekends. I don't know the total count, but by the time they finished building they built a home, a garage, and a sturdy outbuilding. The wife, a civil engineer, learned from her father, who had done the same thing on his place. He built a lot of curved retaining walls and a two-story garage with his. The blocks drove the masons who laid them nuts, as it was hard to set them to a string line, what with the field rocks sticking out of the outsides.