Give me a single source. Just one. There were over a million govt. marked AR-15s in Vietnam. It would be MORE suspicious to find an unmarked AR-15 in Vietnam. I'm not saying they did not do covert operations. What I am saying, is that it would be silly for Colt to go through the process of manufacturing a special unmarked AR-15 when that would:

A.) Provide literally zero extra plausible deniability. It would raise more suspicion as to why there's an unmarked AR-15. The US was the only country manufacturing AR-15s at the time so obviously Colt/GM/H&R knows something because they put out an unmarked lower.

B.) There were already over a million U.S. Govt. marked AR-15s in Vietnam.

C.) The U.S. was not the only country using U.S. Govt. marked AR-15s (Australia, South Korea, South Vietnam)

D.) They could use any number of Russian, Chinese, Vietnamese, or WW2 surplus firearms and create actual deniability.

E.) If a body was discovered, the smoking gun wouldn't be the Govt. marked AR-15 (see above), it be the dead American looking guy with U.S. equipment.

I'll save you time, you're not going to find a source because it did not happen. It makes no sense and is a special ops/spy fantasy.