Tamer 20 bead replacement?

[chipmunk]

 I was looking at my new tamer 20 today as I put it in the case to go shooting tomorrow (can't wait to try it!) and I think I've decided to put one of those fiber optic sights on it.  I tried one on a friends gun awhile back and I really liked it.  The obvious question that came first is "what size is the bead's thread?"  Couldn't find anything in the FAQs so I fired up the search thing.  I found some info on it and I was pretty confident I was looking at a 3-56 since that size was mentioned in several pages, but then I hit this link on brownells:
https://www.brownells.com/.aspx/lid=11183/guntechdetail/Shotgun_Sight_Bead_Thread_List

It lists 3-48!  Can anyone who has done it let me know for sure?  Preferably someone who has done it on a tamer 20 but I can't imagine them not all being the same.

Also, since I'm posting, any suggestions on which ones are good?  I will probably order tomorrow after I pattern the gun to make sure I don't need an odd height bead.

Edit: I must have been a bit tired when I looked at the brownells page yesterday.  It's even more confusing when I read it correctly.  H&R is listed as a 3-56 but NEF is listed as a 3-48.   I find it almost unbelievable that they would thread the NEF marked guns differently. Maybe the pump/auto guns are different? 

Anyway, my gun is marked H&R so I'm going to assume 3-56 if no one corrects me.

[Markus]

I know that used to be correct but with the changes lately I'm not sure?

[gendoc]

in the past, all the h&r rifles/shotguns that i have drilled and tapped
for use with the factory base that includes the screws
has been 3/56... but i can't be sure of "big green's" thread size.