Claes, I think the piston cap is a 400nn........in it's rounded like the 120, 140.
The 400 n's piston cap, is a bit rounder than the 400's...I not having an N, ... do have a problem telling it from a a 400. I once bought a '50-54 400 tortoise thinking it a 400n...it wasn't quite. It was a transition '54 400...nib marked, not piston cap marked, but lacking that bit rounder piston cap.
I finally got a 400nn tortoise EF Maxi-semi-flex...when it rains it pours 400nn's...got two green striped ones, a KF, semi-flex, and a OF Maxi-semi-flex, my first one, and second Pelikan at all... Two of the 400nn's are late last year or early this year.
I got that transition tortoise 400 I mentioned, and the t400nn and a 500 tortoise. I find the tortoise of that era a tad lighter than my 'single' '91-97 400 tortoise (the first of my now 35 Pelikans).
I won that at a live auction, and was so noobie, I didn't know it was '90's thinking it was one of those semi-flex 400's I read about on this com. Still a pretty pen, and I've come to like springy regular flex nibs since then.
Congratulations on repairing it.