I'm considering a Pentax 67 and have come across 'mirror up' in second hand sales. I read somewhere that the first production models (that don't have mirror lock-up) could be modified to keep the mirror in place. Does this mean the viewfinder is unusable!? Or is there some kind of lever or button fitted on these modified versions to move the mirror at will? It's a little confusing and I'm not sure if this term is used by sellers regardless of it being a late or early modified model. The camera I'm looking at is a 67 rather than a '6x7'. Which, if I'm right, means the camera had this functionality straight out of the factory - yet the seller has listed as 'mirror up'.