So I went to Pastime Hardware, one of my local Ace representatives (franchises? I don't know their system - sometimes they go more by their local name than "Ace"). Armed with the not-missing screw and the dimensions from the
Pickup Screws thread, I prepared to dive in. But instead I asked someone, and he looked at it and asked "Is this a 6-32?" I had it pulled up on my iPhone, he was right! Here's the info from the other thread:
The pickup height screws are 6-32 UNC with .5, .75, and 1 inch lengthsHe then asked, "Do you want stainless steel like this one?" and I replied yes, because that's what Dave Houck says in the other part of the thread:
Because ferrous magnetic materials have the most effect on response, we don't use magnetic steel screws to anchor the pickup, we use stainless.I walked out $.30 poorer, but I should have tipped the guy! He measured it for me so that I would get the correct length (.75" if I remember correctly), then went over to the wall of shelves and pulled one out. My bass is fixed! I was going to switch back to the AleM-85 fretless Guild but I think I'm going to stick with the SCSS for now.
Thanks for all the help everybody!
(When I took out the remaining screw to take to Pastime, it was definitely the same screw as the one I saw on the ground at the gig)