Looked through the documentation I have here at home.
The C does indeed stand for the engines in the early production of the 89 SC-the forged crank appears in SCs produced January 30, 1989 and beyond.
So any 1989 SC built between the start of the 89 model year and January 29, 1989 will have the Duracast crank-looks like I'll be ordering 2 new cranks soon, as both of the 89 SCs I own are early production.