Mine is inside the left cup
https://www.bose.com.au/en_au/support/articles/HC1949/productCodes/QCUH-HEADPHONEARN/article.html#:~:text=The%20serial%20number%20is%20located,underneath%20the%20scrim%20(cloth%20insert)
Mine is inside the left cup
https://www.bose.com.au/en_au/support/articles/HC1949/productCodes/QCUH-HEADPHONEARN/article.html#:~:text=The%20serial%20number%20is%20located,underneath%20the%20scrim%20(cloth%20insert)