This Mamiya 645 No. 3-S Extension Tube tube is in mint condition, and comes complete with the original box and a part copy of the instructions – but you can download the instructions from the Downloads section so this should not be a problem. This tube has absolutely minimal signs of use, probably used no more than 2 or 3 times.
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.