This I/X difference is strange indeed. When i booked my RIX LYR, i could get availability in C only, on one leg. The RIX ARN OSL alone was available in Y and C, OSL LYR was not available at all (only Y for domestic norway) but RIX ARN OSL LYR was available in C.

This means that even domestic flights have award availability in C, although there is no C class, and although they don't "sell" C-class priced award with Y seating when booking this flight not in connection with a flight that offers a proper C class.

FYI, i got seat assignment in row 3 on the Norwegian domestic flights together with my international C award booking. (i am EBBasic now).