SWB tandem - the cranked stoker seat tube cannot accommodate a front mech. That's why the chainset is at the front. Originally it would have been a single chainring.
Was it like 4 speed derailleur originally pretty much like @hamster said, but the left hand gear lever used for a drag brake or some weird obscure hub gear? Cable routing seems to suggest this.
Beautiful double diamond frame with bent stocker seat tube.
Perhaps the captain wanted to have a clear view of the front derail trim?
I know from friction bar con days I was always being told “high - low” from my stoker.
I think this is pre proper tandem cranksets. If the rear cranks had a spider for the driveside & the nds then it would have the same setup as a modern tandem would. What an awful thing though. (no offence Jack Taylor)