Its not threaded, but appears to be the pressed-in type with the retention collar. I assumed that all of the older Fats had that type set-up...?? I will check inside the seat-tube, but the post is actually a 27.2 with a shim. It moves freely and appears to not be rusty or seized-up in any way. As far as other rust goes, the bike has a number of small surface paint chips, with tiny amounts of surface rust here and there, so to say absolutely no rust would be incorrect. Nothing significant I can tell you that for sure. For anyone that might be interested the left-side top-tube has the Yo Eddy decal in place, and the right side almost looks like it never had one.
JR
Update Note: Checked the seat-post bore and it looks clean and smooth and the post/shim slid out freely and was well greased. Hope this info helps!
Jeff