This is the shroud for the APEM IB and IS series sealed pushbutton switches. The shroud is added to pushbutton switches to guard against inadvertent actuation.
APEM IB and IS series sealed pushbutton switches - bushing Ø12 mm - momentary
http://www.apem.com/Miniature-sealed-momentary-pushbutton-switches-v9-d-55.html