We all know the theme music for Doctor Who has had the same tune and general sound since the show first aired in 1963, and has been rerecorded several times over that period -- most recently, when Jodie Whittaker took over the TARDIS. I've seen video of someone performing (a version of) the theme on a synthesizer, but synthesizers as we know them (even the Moog and Korg) didn't yet exist when the theme was first recorded.
There's an obvious theremin line for the primary melody, of course, but the rest of the sound doesn't seem to be conventional instruments (strings, woodwinds, percussion, etc.) to my ear.
What instruments were used in recording the original theme music for the show when it first aired in 1963?