It's going to be a code for the date and place of engine mfr.
TO is Tonawonda (New York)- a big GM engine plant
208 is the 8th of Feb (date of mfr)
FCN- I don't know. There are all kinds of codes for the last 3 digits. I would see if a Chevy Dlr can interpret for you. Or maybe ask on chevytalk.com..