Gary,
Nope - stamped right into the bumper, definitely not an ink stamp. First one I had ever seen. I looked at my '69 bumpers, including an NOS one, and did not see any stamping whatsoever. My guess is that the vendor for '68 bumpers, or one of them, stamped their production bumpers: maybe the vendor changed after '68, or changed their process not to include stamping production dates.
Whoever has the car now has the evidence still attached if the bumper is still on the car. Advance Plating in Nashville did the deed(s).
As a sidebar, this bumper went through three plating events. First time, too much copper was used; beautiful job, but soft to the point the guards dimpled the chrome during installation. Redo the second time produced a "blonde" spot where the nickel showed through, third time was the charm. I wouldn't have touched them in the first place if it was my car, but the then-owner insisted on perfection (in his eyes). Pretty vehicle, regardless -