light sockets and thin metal are usually #6 x 3/8"
https://www.titanpinball.com/index.php?route=product/product&path=58&product_id=93
Brackets thicker that the light sockets that don't hold coils, magnets, or anything heavy are usually #6 x 1/2"
https://www.titanpinball.com/index.php?route=product/product&path=58&product_id=94
Flipper brackets, coil brackets, and all the thick stuff are usually #8 x 1/2"
https://www.titanpinball.com/index.php?route=product/product&path=58&product_id=92
BTW, i finally found the correct screws without the slots in the heads. I need to update the pictures on our website