its the quick set date screw. how do i find the sizes
These screws are hard to find, but can be bought from Ofrei. Unfortunately they are like $4-6 but there's a min shipping charge of like $12 so yeah.
I went through my old parts and found one that will work. It's from a different movement but I tested it and it should work fine. PM me your address and I'll mail it to you. Free if conus, if not you'll need to send me postage.
Confirm it's the one in the pic, the date jumper cover screw. The replacement will be blue and has a slightly domed head, but the threads are the same.
Yes you will need to reset the keyless again. Make sure the stem is in the setting position when you remove or insert it. After you get it working again, I suggest removing and replacing the stem a few times to get experience knowing exactly how to do it perfectly on this particular movement. Observe the positions of the setting lever and the corrector lever on the edge of the movement when the stem is in the setting position. That double stack where the arrow is pointing. Thats a properly functioning keyless. It should look just like that with the stem removed. If it doesn't, the keyless is fouled again. Practice removing til you do it with ease every time. Then put the dial and hands back on and remove the stem the last time.