Ultimately, pretty anodyne. Doesn't really seem like there is a story here. It seems more like a reporter made the FOIA request. Not much contentet came back, reporter wrote article anyway.
The short version goes something like: unknown person tells FBI that Fuller has had some contacts with soviet people. FBI sits on the info. After some further contacts and some trips to Moscow, FBI decides to interview Fuller. Fuller is completely cooperative, and talks the FBIs ear off. End of file.
Respect to them for following up and publishing an article anyway though. Just like statistically insignificant experiments, sometimes the absence of a major headline is worth sharing, and it's nice to know that in an era when security services were particularly aggressive in pursuing alleged "fellow travellers", the FBI were able to refrain from persecuting at least one prominent eccentric.
The short version goes something like: unknown person tells FBI that Fuller has had some contacts with soviet people. FBI sits on the info. After some further contacts and some trips to Moscow, FBI decides to interview Fuller. Fuller is completely cooperative, and talks the FBIs ear off. End of file.