Here's an idea: the detective knows that the note was written with a fountain pen, but cannot replicate it, because the original writer has a specialty nib! (Italic, flex, architect grind etc).
I had initially thought of this, but i discarded it because it makes the detective's job very easy which would make it a boring read. I need a detail that makes the reader exclaim in Awe; This is why I began to look into the differences between pressure and indentation of different writers and pens -- That is one thing that has to be unique to everyone, and every pen - and hence cannot be faked very easily.
Myth that tipped nibs change much from....and that takes years of use.....not in days or weeks.
Back in the Day....of One Man, One Pen....a fountain pen's nib would be good for 7-10 years of 8 hours a day, before the nib wore enough to need a new pen.
I think you need a new clue.
Very few people use a fountain pen any more....and there are many who grip it like a ball point pen.
Your detective, could try to write with it like a ball point....which would give a different pattern....from when someone held it behind the index knuckle like a fountain pen.
H'm. I believed the myth because everyone kept telling that 'A pen gets smoother with use' - I thought it made sense logically that constant rubbing on the paper would, like sandpaper does, smooth sharp corners. I was wrong, I see that now because there is no way that paper would wear metal out so easily.
I was looking into selecting something like what you recommended, @BoBoOlson, but other problems arise. Like it said to @KellyMcJ: One thing is that it adds more unique points to the plot - and I want to avoid them as much as possible because they make the Detective's work very straightforward. I have made it so that the detective and the suspect both aren't very well versed with the intricacies of using a Fountain Pen and use a General Light Pressure Ball-Point Hold to handle a fountain pen.
After spending a day thinking about it, Here is what I have learned, and inferred:
Facts: (are these accurate? Do they make sense?)
1. Every Pen has a unique sweet angle, at which it works smoothest ( I feel like the cheap ones like the Hero Hooded-Nib pen, have one)
2. Every person has a very unique way of holding a pen
3. People with a straight handwriting and those with a more sloped handwriting use the Sweet Angle differently.
Here, I see three possible ways to go.
1. Look into subtle differences between Straight and Sloped Writing that would be visible when someone with a Straight Writing tried to Imitate a more Sloped Style
If I choose to pursue this path, I would have to study various samples of Straight and Sloped Handwriting, and finally, attempt (and succeed) at Picking from a selection Which handwriting is Naturally Sloped vs Faked. This skill might turn out to be like that of WW2's Plane Spotters.
2. Writing with the pen the wrong way may leave scratches and impressions on the paper. If someone were to pick up someone else's fountain pen and try to write with it, chances are that he would produce a higher number of scratches and impressions on the paper than the owner ( Is this accurate? ). The lack of such scratches might indicate that the note was written with someone already familiar with the pen.
This one poses another strong problem: The lack of factual Information. How different are the way people hold their pens, really? Aren't most holds very similar?
What are the chances that two people using a fountain pen create similar indentations and scratches on paper?
3. Finally, another important point that I might be able to bank on:
For a sloped writer, writing that way is a normal everyday thing, but for a Faker, Sloped Writing might be more, um, "Signaturial", or Signature-Like. I Conjecture that a Faker might emphasize the long lines in letters like L and F and T a mite more to make them look more natural (or flowy), hence creating a deeper indentation that may be detectable on the backside of the paper.
I'm still trying to form a theory that is both strongly logical and intriguing to the reader. Please critique the ideas I've proposed so far.
And thank you all for your invaluable input. I don't know how to thank you yet, but I'll find a way!