Was just on the USPS web site and looked up "precanceled stamps" and they provided this illustration of possible options that may be used in 2011:

Personally, I don't recall ever seeing a single bar precancel and was wondering how commonplace it may be today? If you want to read the entire ruling it is at this link:
http://pe.usps.com/cpim/ftp/manuals...00/q604b.pdf