This Mamiya 645 No. 1 Extension Tube tube is in mint condition and comes in its original (not mint!) box. This tube is virtually as new and has absolutely minimal 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.