That plate wasn't used in 67 or 68. For those two years they used threaded inserts. The plate was a 69 only thing.
(For 68, check out the AIM, UPC NF2, Sheet A4, item 12 or UPC N10, Sheet A4, item 12)
However, even if the car has the inserts, it only proves it came with dual exhaust, which was optional on both the LF7 and the L30. If it doesn't have the inserts, the car came with single exhaust which was standard on both the LF7 and the L30.