Sanding works!
My adapter was too thick and would not reach infinity.
Rather than sending it back and waiting to see if jinfinance can sort this out with his OEM, I did the following:
Followed Ranger 9's instructions for sanding an adapter that is too thick and therefore unable to focus to infinity. They work.
This link goes to a sample of the best focus possible on some brickwork and roof shingles before sanding:
http://www.flickr.com/photos/41790885@N08/4883074902/
And after:
http://www.flickr.com/photos/41790885@N08/4883074904/
The improvement is clear in these 100% crops (and more dramatic in the entire image). The comparison is not exact: my distance was perhaps 10% different, and the light was different (sunlit before, overcast after). Still, I believe the point is made. Both images were shot at f/8.
The lens is a Carl Zeiss Jena 28mm f/2.8. I do not know what its best performance should be.
The instructions stress the need to sand a bit, test, and repeat as needed. Things are at the point where I don't see further improvement after a round of sanding, but the focus ring does not yet go past infinity.
Of course, I spent a couple of hours on one adapter and might not be finished. The Chinese manufacturer cannot or will not machine the part correctly in the first place. Looking around the Web, all merchants who sell adapters for Praktica bayonet mount to m4/3 seem to have the same product.
Anyone who might ever use an adapter should copy and save Ranger 9's instructions.