pretty positive there is no choice between single or double. they are all double, usually called spherical i think, instead of flat. if they are crowbar lenses, they are double, and you want double anyway
Sten: We like fishing.
Karl: Fishing, hehe.
Sten: And in the winter we like skiing.
Sten: Of course, in Thailand there is no skiing.
single lens: http://cgi.ebay.com/NEW-OAKLEY-CROWBAR-FIRE-IRIDIUM-REPLACEMENT-SINGLE-LENS_W0QQitemZ150360760371QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2302332033&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A2%7C39%3A1%7C72%3A1205%7C293%3A1%7C294%3A50#ht_4785wt_941