These mats are located around the stifle to upper leg.  Keep in mind that not all hair will grow into a mat.  There is also no particular pattern or placement for the mats.  They will form and grow at random. 

A grouping of mats/flocks.

Adult mats/flocks showing where some attach to the skin.

Notice the individual hairs that grow separately from the mats.

This is the lower leg and paw of an adult Berg (hanging over a picnic table).  Notice how the mats are wider higher up the leg and taper to a smaller width as you work your way down to the paw.

Adult mats/flocks 1 to 2 fingers wide.

Adult mats/flocks.  This back side area is very thick!