From the CES Pre-Tech Form for the 2015 Blackhawk Event, a helmet must meet one of these qualification:
M2005, SA2005, K98, K2005 or SFI 24.1, 31.1A, 31.2A, 31.1/2005, 41.1/2005, 41.2a or FIA 8860-2004 or Snell-FIA CMS/CMR2007 or ECE-22.05
It would appear that a 2005 rated helmet is legal in CES, but I would contact Dave to be sure.
Link to Snell web site, standards:
http://www.smf.org/stds
Link to Snell 2010 sepc:
http://www.smf.org/standards/sa/SA2010/sak2010cover_5-18-09.pdf