It looks like one of the sketches is dated "64" for 1964. Space Seed was filmed in late-1966, so, the filming model of the Botany Bay must have been built around that time frame, too. Memory Alpha says:^^ That looks like he seemed to have his concept pretty well thought out from the beginning.
more at: https://memory-alpha.fandom.com/wiki/DY-100_classEffects
- The Botany Bay model was actually designed by Matt Jefferies before he came up with the Enterprise. He described it as an "antique space freighter" and put it aside for a chance to use it in the series. (These Are the Voyages: TOS Season One)
- The effects scenes featuring the Enterprise and the Botany Bay were filmed at Film Effects of Hollywood.