I just looked up the Kerr slings on Google and it showed a whole bunch of photos of the slings and not one had any government inspectors stamps. Most government items purchased from private makers usually carry a contract number or a date. However if the government didn't stamp any Kerr slings, the slings could either be a commercial purchased sling or one purchased by the government from the Kerr company and no one would know if theirs is an official government issued one or a private purchased one.
Ray