Too bad most of the FPN folks won't look in this forum to see this. Their loss!
Macniven & Cameron, Made in Great Britain, Waverly Series No P 5
This is a casein pen (based on smell while I sanded it down to restore the color and finish). Small pen, 4 1/2 inches capped, posts to a nice usable 5 5/8 size. It came to me in a sorry state with a bent nib, barrel thread crack (now sleeved internally), and very dark, brown and crackly on the surface.
I was just so drawn to the pattern I had to buy it and put in some work to restore it!
Unfortunately, I know nothing about the brand, history, or other models they might have produced.