Search the forum for "mourning button" and a few threads discussing the black crepe covered buttons, black cloth diamonds, "Alf's Button" will turn up.
Apologies for not posting a few links to threads but searching, copying, pasting is a pain using my phone.
Edit - added a link:
http://www.britishbadgeforum.com/for...ourning+button
A GMIC thread:
http://gmic.co.uk/forums/topic/28394...comment-268313