Optec
Oops, soory, slight slip of the finger on the last reply. It should have been A352, not A356. Having seen the book, I think I can hazard a guess at the meanings of the letters. A216 is entitled Steel Castings, Carbon, Suitable for Fusion Welding, for High Temperature Service. A352 is Steel Castings, Ferritic and Martensitic, for pressure containing parts, Suitable for Low Temperature Service.
My guess is that the W or L refer to Welding or Low Temperature, C for casting, and the A, B and C for the position in the series.
Bruv