I had no idea of the E26 base until I saw mention on the antique light bulb site...they seemed to think it was of different diameter.
So, I got out some genuine Yankee light bulbs and some Aussie E27's along with both kinds of holder. Well, I couldn't notice any difference in diameter...both screwed into each others sockets just as loose/tight as they would in the 'correct' ones.
What was clearly evident is the US bulbs have a shorter base...with some Aussie E27 sockets the contact at the bottom wasn't made even when fully screwed in. Screwing an Aussie bulb into the US socket leaves more of the thread exposed which could be a shock hazard.
