Ah Ok... thanks!
No problem. Straight from IF reps via email:
Serial number deciphering; digits to the left of the letter, build number; letter stands for the model, single digit directly to the right of the letter marks the year of manufacturing; next two digits to the right of the year digit signify the size of the frame, then if it is custom, an additional "C" to the right of everything, at the end of the serial number, and can stand for custom geometry or custom frame detail. No letter at the end means standard model, as per the catalogue.