No. 4 Silver Overlay
This 900 silver overlay safety filler pen was made in the early 1920s. It's quite large, since it is the "long" version of the No. 4 (MB's standard safety pen line up came in short and long variants). The pen measures 140.5 mm capped.
A few experts were known to make gold and silver overlays for MB back in the day - Maenner, Sarastro, Fend, and others. Some of these companies imprinted their brand hallmark on the overlays (like the "S" on Sarastro overlays), but many pens only had the metal purity hallmark like the "900" on this No. 4 (which means that the overlay is made of 90% silver and 10% alloy, most probably copper).
Many variations are found in these overlay safeties. Some have overlay work all over - leaving no black hard rubber parts exposed. Others have some of the black portions exposed - like the pen featured in this article. Personally, I like the contrast between the BHR section/cap top/turning knob and the silver barrel and cap. Silver overlays came in many shapes, chasing designs, clips, and sizes too. See another beautiful example of a silver overlay MB (link). These pens are hard to find, with Collectible Stars I rating them between 10-11 out of 12 points on rarity. One might think that the solid gold overlay safety pens are more expensive because of the value of gold, but that is not the case - I have found that the silver overlay pens are much more expensive because they are more difficult to find.
The nib on this pen is the correct size 4 made of 14c gold. It draws an expressive stubbish line. The nib is period correct and belongs to pens that were made in the mid 1920s.
MB's safety pens have a pin inside the cap which ensures that if the user forgets to screw back the nib into the barrel before capping the pen then it will push the nib in first - preventing any damage that might otherwise have been caused to it by being pushed against the inner top of the cap. This function is explained in an old 1936 catalogue.
For bibliography, see Resources page -->link
Safety fillers are a wonderfully intelligent filling system if you ask me! The nib unit rests inside the barrel when the pen is capped. After uncapping, a turning knob at the bottom of the barrel forces the inner shaft holding the feed and nib to extend and expose the nib from the section of the barrel. As it extends, the nib unit locks its collar against the section so that no ink can flow out of the barrel. This made it "safe" from ink leaks. And, because of the fact that the nib unit is always submerged in ink, this pen was advertised as never suffering from hard starts!
The video here shows the workings of a Montblanc safety mechanism from a tier-3 pen made in the 1930s. The barrel was custom made for me by Francis Goossens, and it allows us to see exactly how the helix rod and nib unit engage with each other and create a seal with the section when extended, also allowing us to visualize how the nib unit sits in ink and is thereby primed at all times and ready for writing.