Automatic Timing Device - the picture is of an auto advance and retard unit built into a magneto drive pinion. These were fitted on Lucas SR1 Magneto-equipped 350 and 500cc UK Bullets between 1956 and 1959(?), and meant that you no longer had to control the timing manually via a lever on the handlebars. When (cheaper) coil ignition replaced the magneto, the ATD was incorporated into the contact breaker assembly.
Elegant solution Mauri. I thought about trying to modify a chain sprocket version but decided in the end that they were only slightly less rare and that the quickest way to the highway was our hosts.
That, at first glance, does appear to be the very thing. It's an Indian supplier, and I'm pretty certain a number of SR1 mag equipped Bullet engines must have found their way out there if they now have used mags for sale with the ATD still on them. The web site in your link appears to be for trade enquiries, so perhaps they would want you to buy a minimum batch of 500!. A