I have come across a NM type wire in my kitchen project I am not
familiar with. It is a heavy (1/2") cable that is feeding the fridge
and one counter outlet. The issue is the thickness of this
wire...which I have to reroute in the ceiling.

The wire is about twice as thick as current 12-3 NM cable...the
printing on the wire says :
Type NM Phelps Dodge PBX 12-3 - W - G 600 volt

Is this just the way they made 12-3 wire in the 70"s...or is this some
type of "special" wire ? Can I extend this within a properly grounded
electrical box with the current type of 12-3 NM wire ?

Thanks, TR