The LD4B was produced from 1967 to 1981 and all of them had the Chrysler part number on them. They were also available to be purchased through Chrysler dealerships during the entire production run.
Definitely not the case. Popular Mechanics had an article on the 1967 Barracuda rally car - it clearly showed a LD4B with no part number. Also check out EBay - LD4Bs with and without the Chrysler part number.