I think this one takes the biscuit for rarity and mystery. In most respects it’s a perfectly ordinary Mentmore Diploma. What makes it stand out from the crowd is its translucent barrel.
The explanation for this that comes first to mind is that it’s a demonstrator. There are a couple of problems with that, though. So far as I am aware, British manufacturers just didn’t do demonstrators in the 30s and 40s. We are used to ink-in-the-barrel demonstrators nowadays, and the idea of a clear barrel to demonstrate the operation of a button filler seems rather strange. It’s not impossible, though. We can’t discount the possibility.
Could it be, perhaps, a trial of a new and experimental celluloid? Others, like De La Rue, had developed semitransparent plastics. Mentmore was never afraid to try something new and perhaps this was a prototype that never went into production.
Finally, this may just be a freak celluloid that lost its colour and became transparent. This is the least likely explanation. One would expect there to be a batch of tens if not hundreds of similar pens, and they just don’t seem to exist. In my many years of fascination with pens, this is the one and only example that I have seen.
Whatever the explanation may be, it makes an interesting pen. Fitted with a translucent plastic sac as this one is, the amount of ink available is visible. Also, you can see the action of the pressure bar as the button is depressed. That’s not something you see every day!