I have previously written a blog post about the Platypus Pens Model 20, but that model 20 is not the final Model 20. Perhaps I should have called this new one Model 21, but I didn’t. My excuse is that even though I sent one to Michael Lampard for review and showed it to a bunch of people at the Melbourne Pen Show last year, I never released it for sale. I’m glad of that because this new version is better than that old one in several ways. It looks better (in my expert opinion!), feels better (again, I’m the expert here), and works better. And it has a clip.
Why did I make changes instead of releasing the first Model 20? Well, I bought an old broken-down Stephens No. 76 pen on Ebay and when I worked out how it worked—well, how it was supposed to work: it was broken—I realised that I should adapt that system for my twist-fill pen.
Stephens No. 76
The Stephens No.76 (1935–1941) is in most ways a conventional vintage ‘button-filler’ pen with a latex bladder, rather like a vintage Parker Duofold. However, the Stephens pen has a trick up its sleeve: rather than removing the blind cap to expose the button that you push to flex the pressure bar that squeezes the bladder, you unscrew the blind cap part way and then push the blind cap itself to flex the pressure bar etc. Because the blind cap doesn’t come off the pen you can’t lose it and you don’t have to muck around with a loose pen part each time. You can read much more about the Stephens pen and see the parts that make up its interesting mechanism here.
As you can see from the photo, it is a small pen compared to the Model 20, but it is actually about average in size for a pen of that vintage. The Model 20 is a large pen, but not oversized. It is quite similar in dimensions to the Platypus Model 10.
The Stephens mechanism is very effective and, as far as I know, it was never replicated by another pen manufacturer. The Stephens pen company appears to have stopped making pens during or after World War II due the loss of machinery and pen drawings in bombing raids on London.
Whereas the Stephens blind cap is pressed firmly towards the body of the pen to compress the bladder, the Platypus Model 20 works with a twisting motion. The twist is about half a turn and is entirely constrained by coarse four-start threads in the end of the body.
Both systems work easily and feel perfectly natural, but I am going to claim that the Platypus Model 20 mechanism is an improvement on its inspiration.
My Stephens pen arrived with the threads within the blind cap totally stripped and so I printed an insert with the appropriate threads, 5mm double start, and glued it in place. The 5mm is a bit surprising to me as I did not expect and English pen from that time to have any metric components. Perhaps the brass bit was from France…
The Stephens pen also came with the latex bladder hardened and broken into a mix of bits and dust, so replaced that with a new latex bladder. Vintage pens very often need a new latex bladder because the latex degrades and hardens over time.
The bladder of the Platypus Model 20 pen is made of a very strong and stable elastomeric plastic, TPE, and, in contrast to the old latex bladders, it can be expected to work for many, many years without hardening or splitting. It is 3D printed in a shape that minimises the stresses that come with being repeatedly twisted and untwisted.
Spare parts for the Model 20 filling system are available, but are not expected to be needed. I have tested the bladder to 100,000 actuations and found no signs of fatigue or deformation. The threaded components of the Model 20 mechanism were tested at the same time and showed a little wear, but were still fully functional. (These numbers relate to the testing of the final design of the bladder. Earlier tests with different plastics and with slightly different designs failed or showed signs of fatigue with lower numbers of cycles.)
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