I would say your pen is an early series of 400 pen (not M400) issued right after the discontinuance of 100N pen. That will be in the early 50s.
有的认为是比较早的年代,在50年代左右。
I have a pen just like the one described in the thread. (Not seeing the pictures). Judging by the condition of the materials, i'm pretty sure it's from the 1980s rather than the 1950s. The nib unit has a steel collar and seems a tad smaller than the threaded ones.
I have a 400NN with a 'friction fit nib'. Can't work out if the nib collar is stuck in the barrel or if it is meant to be that way. Writes beautifully in any case - a lovely wet and flexible nib.
400NN也有这种插拔尖。