This Mamiya 645 No. 3-S Extension Tube tube is in really excellent condition, and comes complete with the original box and a rather worn original copy of the instructions. This tube has only normal very minor signs of use with slight paint loss on the mount and slight marking from connecting lenses.
This extension tube will give you 0.44 to 0.60x magnification with a standard 80mm lens; a copy of the Mamiya instruction leaflet can also be downloaded from the Downloads section. I tend to use this and the #1 and #2 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.