This is the shroud for the APEM IB and IS series sealed pushbutton switches. The s
hroud 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