Right.  Should be useful for future 703n user. My serial console is broken, it is worthy to spend half an hour to do it because it indicated to me "fast blinking mode" is the fail-safe.  The console for v1.2 box is very difficult to build, so I leave a more complete note here for un-brick without serial console:

1. Press the reset during power up for about 4 to 5 seconds.
2. The LED will start to blink within a second. If the blinking rate is normal, the fail-safe mode is  NOT entered, repeat step 1. Fast blinking will be 4-5 beat per second.
3. Telnet to the box at 192.168.1.1  . No DHCP, so you need to manually configure your PC to the 192.168.1.2 or other IP at that subnet.
4. Get a "squash-sysupgrade.bin" from the snapshot or the formal backfire in future. Get a nc (netcat) for your PC. I prefer Linux because my virus scanner does not like the Windows "nc".
5. At PC: cat XXX.bin | nc -l 3333     <- 3333 is an arbitrary port, any free port would be fine, something higher than 1000.
6. At the telnet dialog:   nc 192.168.1.2 3333 > /tmp/703.bin      Copy would be finished within 5 seconds. Too short or long is no good or meaning failure.
7. Telnet dialog:
    cd /tmp
    mtd -r write 703.bin firmware                  Press "w" repeatedly (or there may be better way) until the box reboot.

8. Cross your finger.