Originally posted by Squire
The difference was the P-51C was built at North American's plant in Dallas Texas.
Its ez to think they were different sub models however, and why they bothered with a "C" designation is beyond me, maybe it had to do with accounting procedures.
The P-51K was a 'D' built in Dallas.

The first P-51B flew on May 5, 1943, and the first P-51C flew on August 5 of that year. Inglewood built 1988 P-51Bs and Dallas built 1750 P-51Cs.
With the introduction of the P-51C-5-NT onto the Dallas production line and the P-51B-15-NA in the Inglewood production line, the Packard
V-1560-7 engine was adopted as standard. It offered 1450 hp for take off and a war emergency rating of 1695 hp at 10,300 feet. Maximum speed at 20,000 feet was reduced from 440 to 435 mph, but increased from 430 to 439 mph at 25,000 feet. 398 P-51B-10-NAs, 390 P-51B-15-NAs, and 1350 P-51C-10-NTs were built, all powered by the V-1650-7 engine.
All preceeding dash numbers of the B/C used
V-1650-3 engines.
For more info on the P-51 for those that don't believe what Squire said:
http://home.att.net/~jbaugher1/p51.html