There are two polymer and two rubber pins in the upper receiver at the interface between the upper and lower receivers.

Does anyone know where these can be purchased? I've searched for hours and have not even found a part number. A call to S&W resulted in being told they are not available separately. Thanks.

I had to open mine up to check. I didn't even realize those things were there.

If S&W can't help you, I don't have an answer. I suppose you could try giving Brownell's a call. If anyone outside of S&W is going to have them, I would think it would be them. If they don't have them, they might be able to direct you to who might.

Out of curiosity, what happened to the ones that came with the rifle that you are looking for replacements?

Bought a bare receiver for a build, and it did not have these pieces. Without them the fit is very loose to the point that it is not an option. A call to Smith and Wesson was useless as they said they are not available separately and they won't send any.