Living near DC had its advantages when I was growing up-We got to go on some pretty cool field trips. One year, we went to the J Edgar Hoover building and got a tour of the FBI facilities and various labs.
Very cool stuff, rooms full of old guns and spy equipment, the lavish possessions of drug dealers including a stuffed grizzly...and one of the areas I remember distinctly is the ink and typewriter forensics (probably now computer/tech) areas. They told us they could figure out which of hundreds of typewriters had been used to type a letter, that they could test ink...It was fascinating to me at the time that it was even possible.
If this diary were to make it in based on the findings of an ink specialist, I don't think there should be any issue with convincing a jury of the science. There are decades of work behind ink studies and the FBI is so familiar with it, they can make the process intelligible (literally) to a 5th grader.
ETA: What are the chances the pen could be as rare as Henkl and only manufactured before or after a certain year as well??
http://www.ojp.usdoj.gov/nij/journals/258/forensic-databases.html
International Ink Library
The collectionmaintained jointly by the U.S. Secret Service and the Internal Revenue Serviceincludes more than 9,500 inks, dating from the 1920s. Every year, pen and ink manufacturers are asked to submit their new ink formulations, which are chemically tested and added to the reference collection. Open-market purchases of pens and inks ensure that the library is as comprehensive as possible.
How does the library work? Samples are chemically analyzed and compared with library specimens. This may identify the type and brand of writing instrument, which can be used to determine the earliest possible date that a document could have been produced. If the sample matches an ink on file, a notation is made in the database. The U.S. Secret Service generally provides assistance to law enforcement on a case-by-case basis. For more information, contact ###-###-####.