Mulviman

Hi all, I've seen on most other singer 111w155's there is a thumb screw just behind this set screw on top of the machine. On mine there is no thumb screw on top, and I'm seeing that this hole is indeed threaded. Looks like it would apply more pressure on the inside presser/walking foot right? Does anyone know the part number and where to find the missing screw? Otherwise I can find a bolt that fits. Here's a pic (my screwdriver is pointing to the hole that I believe is missing a thumbscrew)