Platignum were a Mentmore brand, specialising in lower cost fountain pens that were ideally suited to the student market. Some models 'broke the mould' a little by adding features that were not found on the majority of Platignums.
This Platignum Visi Ink falls into thet category, it has a see through ink 'window' to inform when the pen needed filling and was a piston filler.
The filling system works by turning the knurled button first anticlockwise to expel air then clockwise to draw in the ink. It works very well and the ink window, although slightly 'ambered' through age, is unstained and gives a clear view of the ink level.
The nib is a Platignum '1st Quality', a stainless steel two tone affair that writes well with a medium to fine line.
Platignum produced a huge range of models and it is sometimes difficult to date them precisely, I would think this one dates from the 1950s or early 60s. The Visi Ink is not a common model. It is in excellent working condition throughout and would make an interesting and reliable every day writer.