It's a "Twist Filler" (really a piston), only made in 1943. As I understand it, it didn't sell well because it was overpriced in comparison to the J pens that dealers still had. All of the metal on it is palladium/silver because of war rationing, both the nib and cap trim. These usually don't work and are near impossible to repair but this one works!
It is it between the size of a Transitional J and a Transitional SJ, those are the other two pens in the pictures.
Now I have to decide if I can afford to keep it