The Z568M is a read coated numeric nixie tube manufactured by WF. It is the biggest nixie tube that WF/RFT has produced. It has a enormous digit height of 50mm and was used in applications were the viewer usually was a few meters away. These tubes are perfect for giant sized projects. These tube are not as rare as may thought, they are available in several pieces on ebay, but its high price (usually over 150$/pc) make them 'rare' for projects. There is also a clear glass edition, the Z5680M, and there is a likely replicate by Dalibor Farny, the R|Z568M