Asus Nexus 7 (tilapia)

The startingpoint was an install of Android 5.1.1 by Asus Service Australia.
I became 'Developer' and I unlocked the bootloader.
I have successfully downgraded to Android 4.4.4(nakasig-ktu84p), then downgraded to 4.4.2(nakasig-kot49h).
The bootloader remains unlocked, and being 'Developer' is now a redundant step.

I followed the instructions starting from https://developer.ubuntu.com/en/start/ubuntu-for-devices/porting-new-device/#dev-setup
phablet-dev-bootstrap -c phablet
cd ./phablet
. build/envsetup.sh
lunch
make -j1

At this point I utilised fastboot:
fastboot -w flash recovery /home/jonathan/phablet/out/target/product/tilapia/recovery.img
fastboot -w flash boot /home/jonathan/phablet/out/target/product/tilapia/boot.img
fastboot -w flash system /home/jonathan/phablet/out/target/product/tilapia/system.img
fastboot -u flash userdata /home/jonathan/phablet/out/target/product/tilapia/userdata.img

My query regards this failure:
jonathan@Matryoshka:~$ fastboot -w flash boot /home/jonathan/phablet/out/target/product/tilapia/boot.img
sending 'boot' (8998 KB)...
OKAY [ 1.453s]
writing 'boot'...
FAILED (remote: (InvalidSize))
finished. total time: 1.480s

For experimentation I tried:
fastboot -u boot /home/jonathan/phablet/out/target/product/tilapia/boot.img
This resulted in an imediate freeze.

Is the following a relevent step for my situation?
$ fastboot boot out/target/product/mako/recovery.img
$ bzr branch lp:project-rootstock-ng
$ cd project-rootstock-ng/
$ ./rootstock-touch-install vivid-preinstalled-touch-armhf.tar.gz out/target/product/mako/system.img

I have been sitting on this problem for a few days, but these are the files that were last created:
jonathan@Matryoshka:~$ ls -l /home/jonathan/phablet/out/target/product/tilapia | grep .img
-rw-rw-r-- 1 jonathan jonathan 375702 Oct 1 21:57 android-ramdisk.img
-rw-r--r-- 1 jonathan jonathan 9213952 Oct 1 23:15 boot.img
-rw-r--r-- 1 jonathan jonathan 4426255 Oct 1 23:15 ramdisk.img
-rw-rw-r-- 1 jonathan jonathan 2754570 Oct 1 23:15 ramdisk-recovery.img
-rw-r--r-- 1 jonathan jonathan 7542784 Oct 1 23:15 recovery.img
-rw-r--r-- 1 jonathan jonathan 66685400 Oct 1 23:16 system.img
-rw-r--r-- 1 jonathan jonathan 106792464 Oct 1 21:59 userdata.img