Until 2013, the US Patent and Trademark Office (USPTO) used its own classification, which in some cases deviated significantly from the internationally used International Patent Classification (IPC). Since January 1st 2013, USPTO use CPC.
All patents until 1920 have been reclassified and can be found by using CPC when performing patent searches. US patent publications published after 1 January 2013 shows the CPC. Patents and applications prior to this date include the US classes.