During an encounter with the IKS Gr'oth under the command of CaptainKoloth, one of the ships was disabled, and its cargo was transferred to the Enterprise. The Gr'oth later returned and attacked the second ship, disabling its propulsion systems as well. The ship was later recovered and towed by the Enterprise to Sherman's Planet. (TAS: "More Tribbles, More Troubles")
This appearance of the two robot grain ships was the first on-screen appearance of this ship class. The cargo drone ships were clearly intentionally labeled with the same NCC-G registry in the 1973 episode. It was not until later, when a CGI model was created to represent independent starships in remasteredStar Trek: The Original Series in 2007, that ships of this class appeared with separate NCC registries.
The cargo drone ships were among Michael Okuda's favorite spacecraft designs from Star Trek: The Animated Series, because he felt they looked as though they had come from the mind of Matt Jefferies (who designed the Enterprise). Regarding their configuration, Okuda commented, "It was obviously well thought out and it had a wonderful quality that you could quite clearly see came from Matt Jefferies' universe. It seemed to follow the same design principles as the Enterprise, and yet it was dramatically different. It was my favorite ship on The Animated Series." (Star Trek: The Official Starships Collection, issue 63, pp. 10 & 12)