Issues installing Mesa stock adapter to Rem870

[erasmovito]

Hello,
I'm having an issue with the fitment between the Mesa stock adapter and the receiver on my 870. I'll try to attach some photos to show what I'm seeing.

It looks like I may need an additional spacer or plate, but I'm not sure which one. Also, when I position the adapter where it seems to fit best (without the extra plate), the alignment with the trigger guard doesn't look quite right.

I'm wondering if I may have been sent the wrong adapter, or if there's something unusual about my receiver or trigger guard that's causing the issue. Any guidance would be appreciated.

[Scorpion8]

You may be missing the "stock bearing plate". It is a small piece of flat steel shaped to set inside the rear of the receiver. Numrich or eBay should have one.....(https://www.ebay.com/itm/158004609982)