The number 09261600 refers to the pre-september build A.0095 according to the metadata in the various update packages that I have seen so far. So you would need to start with d1e85d55938fdd545fcdc4b4b11098c5d183636f.zip which has:
pre-build-incremental=09261600
post-build-incremental=10011803
then the october update a1b56846df6bb6c656a511eeae9f732f5b35fbf9.zip with:
pre-build-incremental=10011803
post-build-incremental=11150009
and finally the december update eab40c208c6bd6899b51e7103822972d7a40336c.zip with:
pre-build-incremental=11150009
post-build-incremental=12171325
But this will probably not work as adb sideload seems to be broken, see post #18, since update_engine_sideload is not there, I just tried sideload for the december update and it validated but then failed to install with the same error about the update_engine_sideload. Probably something similar to this would be needed for the FPOS recovery.
PS: There are (at least) two different september updates, which depend on the version that was installed when the device shipped:
- for A.0081 (08161740):
39ce8b1b568149d8ecb8b4b7fc0d37beefc388fb.zip - for A.0095 (09261600):
d1e85d55938fdd545fcdc4b4b11098c5d183636f.zip