Bordoni Transformer Dimmer (1930s)
Siemens

Bordoni dimmers at Beaulieu Theatre, Lausanne
The Bordoni system at the Beaulieu Theatre, Lausanne (opened in 1954) was (in German) a Bühnen-Wechselstromsteller. Which seems to translate as the far less impressive-sounding 'Stage AC controller'.
We're grateful to Emma Davis and Martin Chisnall (who toured to the venue with Mamma Mia in 2016) for the photographs of the system and the documents.