NM wiring question

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

Yes, and even into the mid-80s, three-conductor cables typically included
three "ropes" of twisted paper or polyester fiber spiraled with the
conductors. It gave the cables a fairly smooth surface, instead of the
rope-like surface that's typical now. 14-3 cable used to be about the same
outside diameter that 12-3 is now, and 12-3 was pretty darn fat.

type of "special" wire ? Can I extend this within a properly grounded

Probably not.

electrical box with the current type of 12-3 NM wire ?

I don't see any reason why not. What you describe is pretty well normal for
the older 12-3 cables.

Thanks, TR