Does anyone know why some 149's come with 14K nibs and some with 18k nibs?

It has to do with the era the pen was made. 14K until the nineties, I think, then 18k.

For writing (rather than impressing associates) 14k is the way to go. All 149s, except those marketed in France, had 14k nibs until perhaps 1992 or so. I'm sure a MB expert can fill in the date. the 14k nib is superior for eerything but scrap gold resale.