No. A USB side-loaded book in AZW3 or KFX format with embedded ASIN, and tagged EBOK, is treated as Amazon books. Everything sent via Send to Kindle is tagged P

No. A USB side-loaded book in AZW3 or KFX format with embedded ASIN, and tagged EBOK, is treated as Amazon books. Everything sent via Send to Kindle is tagged PDOC so they behave differently. Normally, books sent via USB can't use Whispersync to sync reading position between different devices. OP is saying that if you embed ASIN into the book (can just be a random ASIN) and sideload the same books to multiple device, they will sync, too.