Just for reference, the WTTID is unique for a given service on a given day. As well as the "visible" part (such as 1G20), there are another four characters that appear on TRUST to aid identification.
So our hypothetical 1G20 on the 6th of the month might have a "full" headcode similar to 1G20MD06.
So our hypothetical 1G20 on the 6th of the month might have a "full" headcode similar to 1G20MD06.