This Mamiya 645 No. 1 Extension Tube tube is in really excellent condition. This tube is not new but has only normal really minor signs of use.
This extension tube will give you 0.15 to 0.30x magnification with a standard 80mm lens; a copy of the Mamiya instruction leaflet can be downloaded from the Downloads section. I tend to use this and the #2 and #3 rings just to get a closer focus with my 150mm and 210mm lenses – it is so small that it is no problem to carry around in case you need it.