I can only guess how that foldable crank is fixed. Most probably it is a plain steel pin (that has to be driven out to release the crank).
At model-craft shops you shall find steel wires of apt thickness. Cut off a short piece, flatten the ends and use it as a driving pin.
Maybe you'll find at your home a tiny nail or needle of apt thickness, then proceed as above.
I just had a more closer look at a photo of a Mamiya 7 II bottom plate; the crank seems fixed different from what I thought above.
Last edited by AgX; 11-16-2012 at 07:33 AM. Click to view previous post history.