A B737-300 crew was confused by a PIT Runway 28C sign; APCH-28; which was the hold short line a great distance from the runway. A runway incursion resulted as the aircraft passed beyond the sign; which was not recognized as the hold short line.

Narrative

We were cleared to taxi out to Runway 28C in PIT from the ramp at V1 taxiway. Runway 28L was closed at that time. While turning left on Echo; I pointed out the 28C approach hold sort markings to the Captain. I then made a statement to the Tower Controller we were ready for departure. He read back hold short of 28C. I responded with the same. I then stated to the Captain while I only saw the 28C approach hold short markings; I did not see the 28C hold short markings; so he probably should hold short of the 28C approach. He seemed confused and I repeated the same statement; saying hold short right here. He then moved just across the 28C approach markings. There seems to be some confusion as to which markings those are on Echo so far from the end of 28C. If the approach hold short is the actual 28C hold short markings; then the signs need to state the 28C HOLD SHORT; not 28C APCH. We were NOT instructed to hold short of 28C approach. We were instructed to hold short of 28C. There may or may not have been an incursion. ATC did not make us aware of any pilot deviation. We were approximately a foot over the 28C approach hold short markings. If this line is the 28C hold short; the 28C approach signs need to be removed. The signs need to say 28C.