It isn't undocumented. There is no serious doubt that such a report was created, and that it was accurately summarized by Dr. Lewy. I have no idea why you keep insisting that since the report is no longer available, Lewy's original summary and citation are therefore somehow suspect.
The NCIS report under question hasn’t been found. No NCIS record of the report has surfaced. No NCIS record of an investigation into WSI has surfaced. No NCIS individuals have stepped forward to say they investigated WSI.
No institutional memory of the NCIS looking into the WSI, as requested by Hatfield, or deciding not to look into it. A spokesman for the NCIS, Paul O’Donnell, has said that they don’t know whether the report ever existed, or might have been destroyed.
No other historian, or anyone at all other than Lewy, has said that they know anything at all about whether the NCIS investigated or decided not to investigate WSI.
And this is a mystery. Both “sides” had a lot to gain or lose from Hatfield’s requested investigation.
Nixon was convinced that the WSI was fraudulent and could be so proved, while the true WSI believers would fear an NCIS cover-up, yet hope for vindication as with the My Lai investigation. Wouldn’t the White House be closely monitoring and encouraging the query? Ditto Hatfield, the Fulbright committee, and the liberal press.
So how did the investigation and its report just disappear?
Was it aborted at conception? Did it take place just as Lewy says, but inside forces loyal to VVAW got to the records and destroyed them, as per our speculation?
It casts no doubt on Lewy’s honesty to doubt the existence of the report. For example, he might have been mislead by a bureaucrat at NCIS who gave him a false summary just to cover up the fact that NCIS dropped the ball and didn’t do the investigation they were supposed to do.
I can think of other possible reasons for why the NCIS report isn’t available, just speculation, and none of those what-ifs would cast suspicion on Lewy’s honesty.
Even the best historians make mistakes. Perhaps Lewy made one, perhaps not. But the fact remains that Lewy's citing "Office of the Director, Judge Advocate Division, Headquarters USMC, Winter Soldier Investigation files" doesn't yield any trace of the goods, when going to that cited source.