Here is a memo that Louis Ivon, an investigator for Garrison, wrote in November 1968. The basic premise is that, because there are differing accounts of Oswald's height, he must have been impersonated at some point. This was always a theme of Garrison's - he believed that Kerry Thornley, a Marine colleague of Oswald, was the "second Oswald."
The memo claims that the first to notice the impersonation was Robert Oswald. "If you really want to know what I think, it is that Robert knew this returning defector was not really Lee."
The last two pages are classic. "Bill Boxley [a Garrison investigator] said that the CIA has successfully put over impersonations so that even mothers are fooled." And, so the reason Oswald stayed away from his mother is not because he wasn't crazy about her, but, yes, she might see through the impersonation. "My idea is that, since she is a very sharp woman and had already noticed the difference in the texture of his hair, he realized that it was dangerous to stay around her. She might at any time see thru the imposture." Indeed, "a lesser woman could be fooled, but not our Marguerite." And, of course, "Marina knew it, too."
You can read more about Thornley and the "second Oswald" in my book, On The Trail of Delusion. Garrison even floated the idea that it is Thornley, and not Oswald in the backyard photos!