Zoltan,
The 'p:cNvPr' nodes are Non-Visual Drawing Properties which specifies non-visual canvas properties. See the MSDN documentation for more details.
I wrote a small windows forms app that will open a power point and display the name attributes for all of the non-visual drawing properties for all of the layouts in the Master using the sdk. Please see screen shot below and link for zip of the Solution.

The code basically does the following:
foreach (var slideMasterPart in PresentationPart.SlideMasterParts)
{
foreach (var layouts in slideMasterPart.SlideLayoutParts)
{
get each of the layouts.SlideLayout.CommonSlideData.ShapeTree.Descendants<NonVisualDrawingProperties>();
and put the name attribute to the grid.
}
}