As a recent fountain pen enthusiast (going on three years), I have made my way through several kinds of pens on the low end of the cost scale. The reigning favorite is the Majohn M2 with an EF nib, if you really must know.
A good part of my love for fountain pens is rooted in the nostalgic past. Through my school years in the 60’s and 70’s, I used many “Sheaffer School Cartridge” pens (SSCPs). They cost a dollar and came with a box of cartridges. My favorite ink was Peacock Blue.
Some time in college, or soon thereafter, my pen box (which included a syringe for re-filling cartridges on the cheap) got left behind.
These days you can buy SSCPs on ebay for about $20. Very fun. You don’t need a cartridge; you can fill the barrel directly with ink “eyedropper” (or syringe) style. Problem is, the factory SSCPs have only (M)edium or (F)ine nibs. These days I have been spoiled, and for small printing and detail work, I really must have an EF. I am a nib swapper, so I’m not afraid of swapping an M out and replacing it with an EF. BUT, the SSCPs seem to have a unique nib size. Many pens these days use standard “swappable” #5 and #6 nibs, for customization. The SSCPs didn’t look interchangeable. So my SSCPs remained in the stable on the inactive list.
Until I saw a guy on ebay selling SSCP “new old stock” XF nibs. Well! I ordered a lot of three, and like the result. Though the XF nibs are not as fine as my wonderful Moonman/Majohn EF’s, the XF’s bring the SSCPs out of the barn and onto the active roster. I love the pen’s slim, smaller size.
And yet. These XF nibs consistently write with a very wet stroke. That almost pushes the XF into F territory; plus I’m sorry for the loss of shading that goes with wetter, darker lines.
So I looked again. Are the SSCP nibs really non-swappable? Today I wiggled one of the SSCP nibs out of the section and compared it to the standard #5s and #6s. Top image is #5. Middle image is Sheaffer. Bottom image is #6. Clearly the #6 would never fit. But the #5 — maybe it is close enough. So I took a #5 EF from my inventory. … MMMMmmmm no, the saddle curve on the SSCP feed is wider than the curve of the standard #5. But c’mon, get a pair of pliers and very carefully squoosh the #5 just a bit flatter. Ah. There. There?
Result: with a “pretty good” fit, the squooshed #5 does work in the SSCP. Well, mostly. Some more fiddling may help, but the SSCP does write with an alien EF nib now. The strokes are not nearly as wet, so the shading of the ink does survive.
And yet — an even better, snugger fit between the nib and the saddle of the feed is desirable. I’ll mess with it some more; probably until it doesn’t work any more at all.
The Presbyteer 
Give up on the #5 nib thing. Only pretty much works, only for a while. And I found a guy selling Sheaffers Peacock Blue, original packaging. So I need wonder no more. (No chance of becoming an adult fave.)
After two months at the top of the roster, these are still lots of fun. I have four. Two of them leak/seep just a tiny bit between the barrel and the section — they were not designed for eyedropper use — and so I added a little rubber O-ring which does the job. Without that seal, you get just a trace of ink on your grip fingers. Though the other two pens have no O-rings and do not leak at all. So there is a bit of variability. The screw threads on the section have a high pitch / threads per inch. That gives them a lot of mechanical leverage: remember they were designed to very gradually puncture the plastic ink cartridges as you screwed the pen together. A little silicone grease on those threads helps discourage leaking / seeping.