Chair base is 1/4" thick at edge.
Seat for rail is 1.3/4" thick.
The ferrule provides a precise location on the timber without needing a close tolerance hole in the chair casting. The ferrule stands proud of the casting to be effective. If frequent tightening causes the screw head to contact the casting, the ferrule must be replaced. Ferrules were originally oak, but modern ferrules are a tough plastic polymer.
Screw top height above timber:
Before tightening: about 3". After fully tightening: 2.3/4" min, say 2.7/8" in new track.
The standard REA chair drawings are available here:
Unfortunately the scan quality is rather poor and some dimensions are difficult to read. Chair screws and ferrules are on page 1. The ordinary S1 chair is on page 4.
Extracting some dimensions from more readable scans:
Screw head top. 1.1/16" sq.
Screw head at flange: 1.1/8" sq.
Screw head height above flange: 7/8" at side, 15/16" at corner.
Flange thickness: 3/8" at side of head.
Overall height of screw head and flange: 1.1/4"
Flange diameter 1.7/8"
Ferrule diameter at top (before compression): 1.35/64"
Boss on casting: overall height: 1.1/2"
Boss on casting: top diameter: 2"
Pictures of some chairs. Notice how prominent the screws can be. The screws are galvanised, the chair castings are raw, which is why the screws stand out in photos, especially when new: