There are many discrete semiconductors with names like 1N4148, 1N4007, 4N35 or 6N137... And they can be produced by many manufacturers. So it looks like it is not a marketing names.
Looks like 1N are diodes of any kind, 2N - transistors, 4N and 6N optocouplers...
How does they appeared hystorically?
What means the first digit and the N letter?
How is it possible that these devices have the same parameters while comming from different producers?
Answers for Numbering of Diodes and Transistors question does not explaining the meaning of 4N and 6Ns and does not explaining the background of these naming conventions. I'd like to have more reading on this. Reference to JEDEC made by winny is very useful. I think that I will be able to find something there.