[GDubya]

Ah... I'm not a Mossy guy (and that's why I didn't speak up earlier), but aren't the 590A1 and SPX barrel dimensions slightly different from the 500/590?

I bring this up because a friend makes shotgun light mounts and he distinguishes his offerings between these models.
http://www.cdmgear.com/lightmounts.html

 

[GDubya]

If the OP is still looking for a barrel/mag clamp, I still have the one that came with my Choate mag extension. I don't use or need it, and I will check the fit and function on my 590A1 and mail it to you for free if interested.

Now that's what The Basement is truly all about: brothers (and sisters) helping each other out whenever possible. {hs#

adcard7, PM me if you'd like a VC Big Dome safety for that recent 870P of yours. I've got an extra NIB and I'd also ship it to you gratis; I've received past considerations from several forum members that I need to pay forward too.