Having messed about with them on three cars, I've come to the conclusion that -
On a 6-volt system LED bulbs are the best idea - but frequently fail to work due to interference from the ignition and/or dynamo.
One of my cars is fine and runs 6-volt LEDs with no problems, but the other refuses to work at all with the dynamo running. To that car, I've fitted a small motorcycle 12-volt battery and use that to run 12-volt, 21-watt standard bulbs that are very bright.
On 6 volts (or 12) it's a really good idea to wire the indicators - live and neutral - directly to the battery on their own fused circuit. This ensures the maximum brightness; a weak earth on 6 volts is always a concern.
With ordinary bulbs it's better to use a a standard electro-mechanical flasher unit with 3 terminals like this:
https://www.classicbulbs.co.uk/products/...asher-unit or, if you need a smaller one:
https://www.classicbulbs.co.uk/products/...y-6v-3-pin
The third terminal allows you to connect both a buzzer and a light. Cab buzzers all appear to be 12 volts - but this one is loud on my 6-volt system:
https://www.classicbulbs.co.uk/products/...ing-buzzer.
Connected to the same circuit as the buzzer is a tiny but very bright LED light that really easy to fit anywhere - mine is just trapped against the switch panel.
https://www.rapidonline.com/KL-6V-60mA-G...gJBnPD_BwE. Or, just search eBay for "grain of wheat 6-volt LEDs".
Rather than the massive switch usually provided with flashers, I've used a small, less conspicuous one:
https://www.vehiclewiringproducts.co.uk/...-on-switch.