Not necessarily...

They are making repros now from Time Passages I think, that have the 6" screw base. They're supposedly slightly marked. As in, it says "repo" or something inside the base, but it is really hard to read and you have to know what you're looking for.

Why they'd make them withthe threaded base I have no idea. There's no need for it in my opinion - displays the same as a flanged base. It only helps the dishonest, especially being hard to read the markings.

Wes