This thread from from FPN mentions the same duridium nib inscription. It's old so many of the image links are dead, but the remaining picture shows a pen that looks a lot like yours but has an odd calibrated reservoir in the middle. I wonder if the OP of that thread had a pen like yours given the conversation. You could always see if they are still active!
https://www.fountainpennetwork.com/forum/topic/113720-no-name-postal-pen/