No. 4 Compressor
Here's a very special pen from my collection - a No. 4 Compressor from 1924-29. This is not your average black hard rubber MB. While it does look similar to the eyedropper and safety BHRs from the same period, it has a rare "compressor" filling mechanism that was used for a very short time by Montblanc. I never thought I'd be able to add one to my collection!
Compressors came in black, coral red, and red ripple colours, in sizes 1, 2, 4, 5, and 6. While the No. 4 is a mid-sized pen in the series, it is quite long. Also, this pen has substantial heft to it because of the brass sleeve which is part of the filling mechanism. The nib on this pen is a period correct size "4" made of 14c gold.
Now, let's talk about the filling mechanism. Pens that hold ink in a sac, instead of their barrel, have the nib and nib unit connected to the latex sac. In order to fill the sac, it first needs to be squeezed, then the nib must be dipped in ink, and finally, the sac must be released to suck up the ink. This is how pens with pressure bars work. But, with pneumatic fillers, there is no pressure bar to squeeze the sac. Instead, air pressure is increased inside the barrel of the pen in which the sac is housed. This pressure squeezes the sac. The nib is dipped in ink, and then the pressure must be broken in some way.
The very first pneumatic filler was a blow filler patented by Seth Crocker in 1901. It required the writer to physically blow into a small hole at the bottom of the barrel to increase the air pressure inside the barrel which squeezed the sac. But, as you can imagine, this was not very convenient. Since then, many iterations of the pneumatic filler have been produced. The MB Compressor is one such iteration, patented in 1923 (link). Here's how it works:
1. Pull and slide out the bottom of the barrel with the brass sleeve, as seen in the picture.
2. Place your finger on the hole at the bottom of the cone/turning knob.
3. Slide the brass sleeve back into the barrel with force. This create high air pressure in the barrel and squeezes the sac. The nib should be submerged in ink at this time. You will see bubbles come out of the nib unit into the inkpot.
4. Release your finger from the hole to break the high pressure in the barrel. The air displaced in the sac forces ink back into it.
And finally, let's talk about this rather lovely clip that was an accommodation available to a range of pens at the time. It was available in different styles and sizes, as advertised in a 1932 catalogue that I have a copy of.
I don't think pens came with these clips, Instead, clients could purchase them separately, as per their preference.
For bibliography, see Resources page -->link