find the screw in the bottom of the cabinet (assuming it didn't break off in the base plate), slightly loosen the other screw so it can be aligned properly, twist the coil stop back into alignment (pressing it tight as possible against the coil), screw the missing screw back in. if the screw did in fact break off, rather than loosen it's way out, let us know, we can guide you from there. But it will be significantly more complicated.

Quick fix if you have the Allen bolt.
Push the coil stop in and jam the bolt in there

Edit: guess Stern has nuts now instead of bolts? Anyway dame thing applies, unless as mention, the bolt sheared off

Edit: ah, optical delusion on phone.

Call your distributor, they will or Stern will look after you. NIB shouldn’t be missing that piece.

Take other bolt out and hit your local Tru value or other hardware store & save yourself weeks of waiting for Stern. I would think if it worked for a few weeks, bolt is somewhere there. If it was missing from factory it would be flipping bad immediately

I would take the remaining bolt with me to Ace Hardware and purchase a few extra. Replace the missing bolt and play on.

Sure, let your distributer know about the issue but don't sit around waiting weeks for a missing screw to come in the mail.

Call a distro over a screw? Wow

This is the screw you need.

https://www.pinballlife.com/10-32-x-38-serrated-slotted-hex-washer-thread-forming-screw.html

You should be able to find a #10-32 x 3/8 screw at your local hardware store also.
The other two screws you found would be used to hold something to the playfield ( screwed into the wood). Check all your mechs to see if any are missing screws.

At some level it is the principal of the matter… you just spent $6,000 on something you expect to have all the parts included. Sure you may be able to go buy one somewhere else but why should you? If your distro is local and you can get one from them with the same effort as going to the hardware store why wouldn’t you? If they can drop one in the mail just as easy as ordering from Pinlife why wouldn’t you? They left a part out on a brand new luxury item. The distro should say sorry about that and get you one.

Do both if it isn’t convenient to work with your distro to get one quickly, go buy one and call to get one sent to you. To have an expectation of a distro to support their product be a bad thing I just don’t get at all.

Contact Stern's support and they will send you one via mail.

I’m not waiting weeks for a 10 cent part that I can get at any hardware store in 10 minutes over principal.

TOTALLY agree with you; this just aint that level.