That would be a 1st Generation Vacumatic Junior in Mottled Grey. It should have been produced from about 1934 to 1938 IIRC. There may be a two digit number after the vacumatic imprint which will accurately date the pen, but it could have been worn off. It should have nickel (silver coloured) trim unless it is worn in which case it would be a brass color. The filler will be a lockdown aluminum filler unit which will require a small push and turn to release to where it pops up for filling.
To fill the pen, release the filler plunger as described above, immerse the nib in a bottle of ink and repeatedly push the plunger until there are no more bubbles. When the bubbles cease, push the plunger back down (while the nib is still in the ink bottle) and turn it clockwise to seat it. Replace the blind cap (which covers the plunger) and you should be good to go.
Now, standard practise is to have the diaphrarm (little latex part inside the barrel) replaced so that you know it will not let go and pour ink all over. Easy enough to do with the correct tools, but if you do not want to invest in the tooling, send it to a restorer.
Ron Zorn and Richard Binder among others are usually the first people who are mentioned as restorers in the US. I do these as well, but if you are in the US you might not want to send the pen to Canada for an overhaul. Reading your post again the "somet" comment also brings up that you might be in the UK. You might have to search for restorers there as I am unfamiliar with them.

Cheers,
Sean Gosse