Whoa! It is not a signature, its a full book! LOL! I think I have seen this in the past, but I thought it was some garbage at the end of the file. I compared 34xx v350 with v360 and it is a very different garbage which does not even look like garbage. So it may well be a 256-byte signature.
If you have seen it in the dissasembly, that just means we need to bypass it in the boot.bin or X-loader.bin, right? Have you tried to do so and flash a gcd without the signature? Or, do you need to flash the modified boot.bin first with updater.exe before you can update with a regular gcd?