I really don't know the answer to this question, and a search of the Fountain Pen Network site did not reveal any useful information.
A check of a selection of fountain pens (the random daily carry pens on my desk) shows no discernible pattern to these numbers. I did note that some fountain pen feeds don't have numbers, while others do. Also, it seemed to me that in those that had numbers, the numbers appeared to be very similar in appearance - leading to the suspicion that the feeds could have come from a common source that used the numbers to categorize the feeds in some way. Also, I noted that the feeds produced by vertically-integrated and branded manufacturers (Lamy, Cross, Waterman, etc) don't have numbers, while an off-brand pen that I know was made by Cross does have a number.
So I'm also curious. I will post this question on the FPN forum to see if someone there knows.