KevinH passed on some information that he discovered on-line. In order to avoid possible trouble for the source of this information I won't be too specific. The main points that I picked out are:
- Yellow Jersey was a project to bring interactive magazines to Fire tablets.
- It included PDF to KFX conversion.
- KFX does stand for Kindle Format 10.
This information explains why there are references to 'yj' and 'kfx' in Fire OS code going back to late 2013. It wasn't, as I had guessed, an early test of KFX for e-books, but actually the roll-out of magazine capability.
Perhaps the re-purposing of KFX as an e-book format was a later idea and not the original plan. Re-purposing existing software is much easier and less expensive than developing something new from scratch. Some the the quirkiness of KFX and its limited availability across Amazon's e-book collection may be due to limitations in the format carried over from the original project.
The mention of PDF as a source is a bit disturbing. The hidden contents of KFX is probably very unlike previous e-book formats.