You may be lucky here, did a search for 6-m on the Championsparkplugs.com site & here's the result..
http://www.championsparkplugs.com/results_cross.asp?pid=6-m&x=36&y=1
or there's similar on ebay, bit old though..
http://cgi.ebay.co.uk/VINTAGE-SPARK-PLUG-CHAMPION-REG-U-S-3X-PAT-OFF_W0QQitemZ280032483708QQihZ018QQcategoryZ74722QQrdZ1QQcmdZViewItem#ebayphotohosting
http://cgi.ebay.co.uk/2-VINTAGE-CHAMPION-Spark-Plugs_W0QQitemZ260036575371QQihZ016QQcategoryZ74722QQrdZ1QQcmdZViewItem#ebayphotohosting
http://cgi.ebay.co.uk/Vintage-Champion-Sparkplug-No-4_W0QQitemZ290033435221QQihZ019QQcategoryZ74722QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.co.uk/VINTAGE-SPARK-PLUG-CHAMPION-8-COM-LISTER-D_W0QQitemZ160033414076QQihZ006QQcategoryZ9903QQssPageNameZWDVWQQrdZ1QQcmdZViewItem#ebayphotohosting