Lot ID's are an official TTB requirement on all barrels whether received or filled in house, you can find information on Lot ID generation here in 27 CFR 19.485. The Lot ID pattern outlined in the CFR is based on the date they were filled, the basic structure is Digit Year-Letter representing Month (A-Jan, B-Feb, etc)-Two Digit Date. For example, barrels filled on January 3rd 2018 would be: 18A03.
If filling more than one lot in a day (constituted as filling from more than one vessel or filling more than one type of spirit in the same day) you will add a letter at the end of the Lot ID starting with A and moving sequentially thereafter, i.e. 18A03A, 18A03B.
Click here to navigate to see our article on how to edit barreled batch information.