I'm not at that computer now, but I used the pieces command:

0xFFFF -p (part-name)

This seemed to work for initfs but failed with the other parts. In particular, for the main rootfs filesystem, the flashing process began, the hashes would increase to about 6%, then the bar would quickly complete and 0xFFFF would complain of a possible checksum error.

Since the same behaviour appeared with flasher-3.5, I wondered whether the problem is on the NIT rather than the restoring laptop.