The first thing I would look at is the edges to see if it has the factory rolled front edge from being guillotine cut or a if is it was hand cut with square edges from a X-acto knife.
Did you search Topps Vault?
Also here is one listed on the E-Bay. Something to compare.
http://www.ebay.com/itm/1968-TOPPS-5...-/350430255607