This is a Danish Big Ben, unknown model.
It’s a piston filler with an aluminium nut, visible in the photo below. Although it’s not immediately obvious in the photos, there is an ink window that has darkened slightly over the years. The blind cap is missing a gold band. The nib is slightly flexible and is in excellent condition.
- Nib: Big Ben 14K 585, Medium semi-flex
- Nib size: #5
- Cap: Postable
- Length, capped: 129.2 mm
- Length, uncapped: 118.9 mm
- Barrel diameter: 11.8 mm (max)
- Cap diameter: 13.1 mm (max)
- Mass: 18.5 g (empty)