I found this, of course, at an antique store. The celluloid of the cap and blind cap are gray/silver and black, and the barrel is gray/silver and clear. The nib is terrible, and after this pic I swapped it for a FPR flex. There is no branding anywhere, and I'd really love to know who made it so I could find some of their other pens. In my experience, cheap pens typically use lever fillers. So, this is an unusual one for me.
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/
That's so interesting, it sounds like he did! Reading through that thread, there is some interesting info on third tier pens. Thanks for that!