So, these are some additional pictures of my blue-grey platinum 25.

As you can tell from the photos, it's a push-knob filler, and as you can tell from the picture below comparing the size to a Masterpiece number 12, it is a medium sized pen. You will also notice, that the nib is not the one, that the pen was probably sold with from the shop in the late 40s. The size of the nib fits the pen alright, and since it has arguably been like this since the 50s, I have decide to leave it as it is, although I have a 'correct and original' replacement nib among my spare parts. I might change my mind some day, though, and change the nib!