One of the more intractable problems facing the fountain pen restorer is the over-and-under feed. Considering that it is just a forked piece of rubber, it works better than might be expected, sometimes with the help of a piece of twisted silver wire in Swan eyedroppers.
The difficulty is that, delicate things to start with, the passage of a century or more with all its vicissitudes means that many are broken. There are no replacements except from other pens. Cannibalising parts from later black pens to rescue patterned ones is all very well, but all these century-and-more-old black hard rubber pens are precious and I believe that it is wrong to sacrifice one to save another.
It would not be all that difficult to reduce a redundant, more modern feed to fit the small aperture of these pens, but there are two difficulties. I would have no reliable means of splitting the feed so that it could perform the over and under function. Secondly, these old feeds are flexible and would appear to be made from a different grade of rubber from later feeds.
What is to be done? Though it would take some experimentation, I believe I could shape a later feed to fit under the nib and fit the narrow aperture of the section. The original feed channels would be retained. The pen could then be used for writing once again but it would lose originality. Collectors might say that it would be better to leave the pen as it is. That’s a valid point of view but many of these earlier 20th century nibs are crying out to be used. They are beautifully made, precise and often flexible, for those who value that attribute.
Of course I have not exhausted all the possibilities of producing replacement over-and-under feeds. There may be other materials or sources of soft rubber that I am unaware of. I have confined myself to things that I can do. Others, with greater skills and ingenuity might come up with a better solution.
Goodwriterspens's Blog 
Could be a good use for 3D printing, is a suitable plastic could be found.
That idea has merit but I confess I’m out of my depth with modern materials.
possibly like collectors of most things, the purists may well turn up their noses at the suggestion of substituting a later feed in place of one with original design – but if writing is your thing then what others think is probably unimportant.
Not that I shun pens with such feeds, but they seem rarely to come my way, though I’m sure there are dedicated collectors, who have draws full of such pens. The only examples I have with such feeds are two Onoto piston fillers, and an early Swan e/d. Whilst the Swan pen is complete with undamaged feed, the Onotos are both minus their caps and one feed is missing the forward 3 mm of the ‘over’ part, though this may not affect its performance.
For some reason I can’t get into these older Onoto pens – perhaps it’s the more complex repair/servicing required, but whatever it’s a shame and I sense that this view is shared by others – the ‘book’ seems to have fared less well than those for other brands.
If you want these ‘capless’ pens with their o/u feeds, Deb, let me know and they will be on their way.
Swan on the other hand is a brand that is very collectible whatever their age or condition.
Sorry this doesn’t offer a practicable solution to your question, but the suggestion of cannibalizing incomplete pens such as those mentioned above, would not in my opinion, make you a philistine.
I can well imagine that to reproduce o/u feeds, commercially, would be expensive, though some folk will pay anything when pursuing originality.
I think that’s a fair precis of the situation. It’s the earliest over-and-under feed pens that are the most trouble to do anything with if the feed is missing or broken. Later ones can accept a ladder feed that has been tailored to fit. I would be interested in those pens with no caps. That’s very kind of you.
The more I think about it the more I conclude that making replacement O and U feeds is impossible for me. I don’t have the right material. My husband has been having a discussion about these feeds in FPN. I tell him he’s casting pearls before swine.