I have been wondering why 120V and 230V countries have different threads. Is the reason just historical inertia or design based due to different voltages? I found out that the E26 and E27 threads for full sized bulbs seem to be designed for different voltages, the E27 having a larger insulator to cope with 230V.

What about the smaller threads such as the E12 and E17? I have not been able to find anything on these threads. I have lots of E17 lamps at home (Christmas lighting I run on a stepdown transformer.) When when I compare those to a typical E14 socket I can't see a difference in the size of the insulator so I believe technically they should be as good as an E14 socket, they are infact beefier.

I've also found E12 lamps being sold for 230V (Amazon/aliexpress) so I believe those are definitely OK for 230V unless there's something shady going on.

This is idle curiosity on my part, arising from having and handling a lot of American 120V Christmas lighting (I like it) and running it all on step down transformers. I don't really have any plans to try and run it on 230V, it's safer to run this stuff on 120V.