What is this??

[shooter71]

the opening on the stock, what is that ??

http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46285

 

[TrapperReady]

I believe it's a holder where you can stash at least a couple extra shells.

Here's a link describing one. Although the one in your picture is shows the opening higher.

 

[Hoppy590]

its called a speed feed stock. its gor a spring in it and normialy holds 2 xtra shells. its kinda like a pez dispenser for shotgun shells

it works like the plastic coin holders in a car cetner consol, just a simple tube and spring

 

[jrfoxx]

Hoppy590 has it right.My 590 came with the speedfeed stock.It's pretty handy IMHO.Mine holds 2 slugs on each side of my HD gun.