I believe that NAT indicates the natural ivory color of the cloth covering. C.R. may mean cross, and like you suspected, T.R. means tracer. On the 1950-51 wiring diagram in the shop manual I have there are no periods between letters and that dimmer switch wire is labeled DIM. SW. FEED 12 GA. NAT BLK & RED CR. TR. I interpret that as as a natural cotton colored wire with black and red crossed tracers. A replacement for that wire is available HERE.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.