Ok this one is pretty obvious but I have not think about it until now.
The first message has two series of numbers the encoded in the X and the 347 final part, maybe that's the user and password and we just need to properly decode both.
3 4 7 9 12 16 30 37 43 48 51 55 59 65 - 3 4 7

Anyway the underscore hint doesn't fit in this reasoning but I think it was worth sharing.

I have been also working on pentesting the server but haven't found any relevant information aside from what I have shared previously.