Batch No. or Lot No. is a 4-5 Digit/Alphabet combination (e.g. T22F1, 22B1, or T22J1) printed somewhere on the product. The "somewhere" is usually as follows:
1. on the bottom of the product;
2. on the inside of the Headband, just above the Earcups;
3. on the in-line controls or on a Tag near the 3.5mm AUX Plug;
4. on metal head of the adapter;
Above are some examples that'll give you an idea of what the Batch/Lot Number looks like and where to find them. The print for Batch No. or Lot No. is usually quite small; please consider using a magnifying glass if it's hard to read.