I also found the "iridium point" comment a bit odd.
Then I figured that this dialog may have been taken from one of Conan Doyles sixty works (fifty-six short stories and four novels) about Sherlock Holmes. After all, Conan Doyle wrote these stories up to 1927 and the Parker Duofold was introduced in 1921. I could then properly imagine Sherlock examining the writing on an envelope and concluding that it was different from the usual dip pen of the time, but more of a new fangled "iridium point" pen.
"Elementary!", I said to myself.
However, the events in Conan Doyle's works occur up to the year 1907. So it does not seem plausible that the dialog was taken from one of his stories. The only logical conclusion is that the writers for the TV series did not do their homework.
"Obviously."
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