Thanks for the reply, it would be nice if a US owner could chime in to confirm whether the jack is 3 or 4 pole but I am beginning to believe that you might be correct in assuming the US ProntoNEO is 3 pole as well and even if not, from the diagrams I've seen the 4th pole would just be a second ground connected to the same DB9 pin so I don't even know if it would matter.

I think the next step might be trying a USB-serial adapter and then possibly ... crying? I'm beginning to worry that the serial connection on the Pronto itself might be busted.