need help

[stilker]

I need a new sight bead for my special purpose 870. I think this is what I need.

http://www.brownells.com/shotgun-parts/sights/front-sights/sight-beads-prod1886.aspx

I want the white bead. But there are many sizes to choose from. I need help picking out what i need. The gun is early 90's old school if that means a certain size.

thanks for any help