I got this stock from Brownells. Am I missing something or do I have the wrong parts?

Looks like the wrong buttbad.  I'd call Brownells.

I got in touch with a helpful tech. at Brownells. He said Safariland/speedfeed do this all the time. Its mismatched parts.

Definitely wrong buttpad.  Should have two circular recesses that the ends of the magazine springs fit in and it will pretty much butt up against the rear edge of the stock body.

Safariland made it right. They are sending me the correct buttpad.

When you get it, please compare length of pull to the factory composite stock? I've gotta get some relief, and the Mossberg catalog shows 14 1/2 length of pull for the factory composite stock and for the speedfeed models, 14".

Brownells says (SFII) buttstock is solid; duplicates weight and balance of factory wood stock. Both have matte, flat-black finish. Pebble-grained wrist and short, deeply grooved law enforcement (LE) forend.

Mossbergs catalog shows woodstocks for the 500 12 gauge are 13 7/8.  This would cure my stock is too long problem.

Yes, they do.   I got 12 SF II stock sets for the Remington 870.  The rear stock was correct on all of them but five had Mossberg forends.  I simply called Amor Holdings and they pirority shipped me five SF LE Remington 870 forends.  I gave them the production date and code so they could even track the person who made the error.

Mistakes can happen and how a company deals with it matters.   This company corrects it fast.  I still have to call in about some wrong stock bolts and I know they will make it right.

what? you cant put that together?

I assembled one of these stocks over the weekend and had one follower in the spare carrier tubes completely stopped by a significant amount of flash (casting mold ooze out at the parting lines); specifically a circle of flash where the followers exit their tubes in the middle of the stock.  When only one shell was remaining in the tube there was just enough excess flash to hold up the follower from reaching the designed stop; this last shell would simply fall out as there was no longer any spring tension pushing against it to lock it in place.  When properly seated (no shells inserted) the followers should protrude from the openings on the sides about a 1/4" (the flash in the problem tube on my stock was forcing the follower to sit flush).  To resolve the issue I used a 3/4" half round file to remove the parting line flash (sharp X-Acto knife, 3/4" wood dowel fed from the back of the tube wrapped with some medium grit sandpaper, or a round file would work too).  Hopefully your stock does not have an issue, but I thought I'd mention it as something to check while you wait for the correct buttpad.

Thanks. I checked and found nothing that looked like it would cause a problem. I did shave down some rough surfaces though.