This is a good article on Petzvals:
http://www.antiquecameras.net/petzvallens.html
I have examples of both standard and Dallmeyer Petzvals. They are both good. I'm not sure I can say one is better than the other, but the industry generally shifted towards the Dallmeyer arrangement with a few holds outs (Hermagis being an important example. I have a Hermagis 250mm f3 that is one of my favorites. It and the Dallmeyer 3B are the last two I would part with.) It doesn't take too much to go from the Dallmeyer mod to the Rapid Rectilinear (which he also introduced in 1866.)
In terms of soft focus (or diffusion control), there should be an index mark on the innermost of the two cells in the back. Some examples have numbered hash marks on the outer cell/barrel, some don't. Outer barrel screwed all the way home should be the sharpest, and unscrewing introduces diffusion.
On my B&L 2A the rear cells were in reverse order and the front glass was flipped around in the barrel. Just about every petzval I purchased has been messed with.
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