It was recorded in July 1964 at Salle Wagram, Paris, a former ballroom built in 1812. Described as having an “acoustique exceptionnelle”, it was used extensively by EMI at the time, usually with the engineer Paul Vavasseur, who was the engineer on this recording. I don’t know the microphones used – I imagine the usual valve models of the time -- but I suspect that the quality of the recording is primarily due to the acoustic and microphone placement, and an engineer who knew the space intimately. And it was Callas! She also recorded Tosca there in the same year, though her earlier 1953 recording made at La Scala is probably better known.
Salle Wagram is still in use today as a recording and performance venue, and there’s a website with a brief overview of its history (in French). You can even take a virtual tour:
http://www.salle-wagram.fr/
Bon Chance
At the time, it was often the case that orchestral recordings would be in venue’s other than the orchestra’s regular concert hall, many of which did not have particularly good acoustics, or lacked the facilities required. The Vienna Philharmonic frequently recorded at the Sofiensaal (also a former dance hall), even though the Musikverein has a wonderful acoustic. I strongly recommend “The Golden Ring”, the BBC film of Solti, Culshaw & Co. recording Götterdämmerung at the Sofiensaal. Fascinating stuff.