hi kunarion,
Thank you for such a detailed reply! I didn't take the item, as I thought it unfair to the owner given that I didn't know what to do with it. I did take photos, however, and the tracking number is printed so no confusion as to what is written.
I found the proxy in GC69459 Sean's Dinosaurs.
I haven't come across a tracking code beginning GM before, and would expect GC, TB or GT...