This Mamiya 645 No. 1 Extension Tube tube is in near mint condition. This tube is not new but has only really minimal signs of use, just a few faint marks from connecting lenses.
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.