Hi Darloman,
If your tax disc holder thing refers to Bicycle Victoria, Australia, we need a bit more information. Bicycles in Victoria have never had to pay a road tax, have never had to display a tax disc, have never had number plates, and the State of Victoria didn't use the numbers first letters second format (239 XMV).
To top it all off, we stopped using an English style tax disc holder way, way before 1967. Vehicles instead displayed a rectangular decal on the windscreen or in the case of vehicles without a windscreen, tractors, trailers, motorcycles, etc, you bought and fitted a rectangular holder. Even that is now gone.
Nevertheless, Bicycle Victoria is an active organisation that promotes cycling, provides bicycle insurance and generally looks after the interest of cyclists. Maybe they had some system back then to show you'd paid your insurance or to identify your sweatwheel if it was stolen or somesuch.
Tell us a bit more or put up a photo.
Cheers,
