^ Same here on my Slayer; the protruding screw head serves as a limit gauge of sorts. The original screw had a taller and squared off head whereas a subsequent part upgrade brought a thinner head thickness with rounded shoulders. Just an FYI/FWIW.

A question for HB reviewers - Does the brew gauge reflect line pressure when performing plumbed-in pre-infusion? (I am assuming so, just wanted to confirm!)

As I recall, with the machine off and cold it does. Once you power it on for heat it will read high due to the water expansion in the closed hydraulic circuit. I have gauges on my pressure regulators and adjust based on their reading.