On August 15, 2019, USTR issued a pre-publication version of the Federal Register in which the formal announcement regarding China 301 Tariffs List 4A/B will be made. In that notice, USTR clarified the September 1, 2019 effective date refers to the date of entry or withdrawal for consumption for the goods on List 4A/Annex A. Similarly, for List 4B/Annex C, the December 15, 2019 effective date is also based on the date of entry or withdrawal for consumption.  Oddly, the Federal Register notice also includes Annexes B and D which are intended for “informational purposes” only, with Annex B providing information regarding the goods on Annex A, and Annex D serving the same function for goods on Annex C.

See https://ustr.gov/sites/default/files/enforcement/301Investigations/Notice_of_Modification_%28List_4A_and_List_4B%29.pdf for the announcement.