A flick up illuminated finger operated from a left - right switch in the centre of the steering wheel and mounted on the door pillar. Had them on my 1936 Lanchester "Roadrider", bought with WWll de-mob money. Lost one when turning too near a gatepost.
My dad had trafficators fitted as an upgrade to his 1954 Ford Popular, PPT482. They came with the neatest little mirrors fitted in the top corners of the windscreen so he could see if they were working - or not - and it was my job to thump the B pillar if they didn't. He thought they were the dog's whatsits but complained bitterly about the vacuum-operated wipers which always seemed to be needed most when going uphill!
My 1954 Morris Minor had these. A party peice was for them to get stuck in the 'out' position, and when you got out of the car you bumped against it and snapped it off. Devil's own job to replace. I think a simple electro magnet coil drew down a slug of soft iron when you operated the switch, which pivoted the finger to the horizontal, also making a connection so the bulb lit up.