503 stands for 50k Ohm (50*10^3)
"C" I think points to that it's a logarithmic potentiometer (?)
EDIT: The best is to measure it. With a Digital Multimeter you measure the outer two pins (should be approx. 50k Ohm).
Put the potentiometer in the middle position and measure the middle pin towards the two outer pins. If the resistance is approx. 25kOhm, the potentiometer is linear. If there is a big difference, the potentiometer is logarithmic.