You need these screws to mount the base of Eames fiberglass chairs to the shock mounts (rubber disks) under the shell.
Eames Plastic Chair Screws Metric Inch Thread Shell To Base
Set of 4 screws for Eames chairs

They have all the same diameter, the sizes show the length of the screws! Length depends on what base you have.

12mm | 1/2" for H-Base | Eiffel Base | Dowel Base | Stacking Base
16mm | 5/8" for Tandem Bench and Office Chair base with Casters
18mm | 3/4" for LaFonda Base

Usual all Herman Miller and also early Vitra production (also with Herman Miller label) used inch thread, all Vitra production (in most cases labeled with a relief-like "V" underneath) need metric thread. Please make sure what you need before ordering. Metric screws have 10 thread turns on 1 cm.

Not sure if metric or inch? Please read this https://www.design-restoration-spares.com/inch-or-metric