Summary
Can I use a Mamiya extension tube with a Phase One camera body?
Yes - the extension tubes named NA 401, NA 402 and NA 403 can be used together or separate.
In-Depth
They are identical to the Phase One extension tubes
Note that AF is not supported when using the extension tubes. Speed and aperture will although be possible to set on the camerabody. Samples images below made with a 80 mm Phase One lens and all 3 tubes mounted on a Phase One 645 AF camera. Note that the 12 micron pixels on this 6 mp sensor are nearly visible on the 2nd image.