Upload & Sell: On
Here's an Auto Mamiya/Sekor 21mm f/4 SX lens that I just received, and a sample image from a 5DII, 1/800 sec, f/8, ISO 200. It's very sharp in the centre, less sharp at the edges.
The first photo shows the 21mm f/4 SX mounted on a 5DII, followed by an example full image, resized to 800 x 533. The next photo is an edge crop from a 2000px x 1333px version of the same image, to illustrate how the edge region looks on an image sized for display on a large, high resolution monitor. The third image is a 100% crop of this edge region. The entire image would proably be just fine for 8" x 12" prints, but the edges aren't so great when pixel peeping.
The 5DII mirror hangs on the shroud around the rear element for subject distances of from about 5m to infinity (about 1/3 of the focus ring range). The mirror releases fast & easy when you set the focus distance to about 5m.
The Mamiya/Sekor SX mount has an aperture ring overhang, or raised edge, of about 1.5mm that has to be ground off before you can get infinity focus on a standard M42 adapter, as shown in the last photo.