I'm trying to restore the original firmware on a wndr3400 (v1) from chaos calmer (Bleeding Edge, r43977)

I've tried the instructions at kb.netgear.com/19841/Reinstall-the-firmware-on-a-router-without-the-setup-CD-recovery-tool, but the router doesn't seem to go into TFTP mode with them.  Even with reset held in, it seems to boot up and I can ssh to device.

Next I used the info on the generic.uninstall page but am not sure about the format.  The netgear file is a .chk and the command fails.

root@OpenWrt:/# mtd -r write /tmp/WNDR3400-V1.0.0.52_20.0.60.chk firmware
Bad trx header
This is not the correct file format; refusing to flash.
Please specify the correct file or use -f to force.
Image check failed.

Is there some conversion that needs to be done, or is forcing it okay?