McCoy spent a few months on the planet Capella IV ('Friday's Child'). Judging from the uniform he's wearing in the screencap
, this must've been what he was doing immediately before joining (or rejoining, depending which writer you believe) the Enterprise
crew in between 'Where No Man Has Gone Before' and 'The Corbomite Manuever.'
I tend to disregard the uniform, since the episode seems to suggest it's been a longer time since he was there. I think it may fit best as part of the same medical relief mission that took him to Dramia in 2251.