I do not know. They are 6mm metric acorn nuts (but I don't know if they have a special depth). When I lost a couple, I replaced them with two small nuts, sistered up and locked together (with copper antiseize). You can't see them (on the inside) unless you are laying down.
Procaliber
NUT, CAP (6MM)
90304-438-000
$4.14
http://www.procaliber.com/oemparts/a/hon/506cb6a8f870023420a41910/muffler