i have a 149 Mont Blanc fountain pen that i believe is a broad nib. However, most of the nibs that i have seen have the numbers 585 engraved on the nib at the base close to the barrel. However, this pen has the numbers 750. Does anyone know about this or know of any other Mont Blanc nibs with that number? In all other respects the nib looks the same as any other Mont Blanc nib from any other pen i have, other than its size... which is quite large.

The number is the gold content, expressed as a fraction of 1000.

So 585 is 58.5% gold content, or 14k.
750 is 18k gold.