Please remember that the words "Ancient", "Weathered", "Modern" and "old" are alll the same Tartan. It is the weavers way of offering the same Tartan in different color variations that some customers may prefer and the weaver can sell more fabric.

Any time I wish to research a Tartan my first stop is always The Scottish Tartans Authority.

http://www.tartansauthority.com/web/site/home/home.asp

This site was until the introduction of the Register the world wide authority on Tartans. There you will be able to look through all the registered Tartans for a given name, see the actual thread count, and see a computer generated picture of what the Tartan looks like.
The STA does not list "Weathered" or "Ancient" versions if it is the same Tartan.

As the "Official" Scottish Register of Tartans comes on-line and is fleshed out it will become the single source for researching Tartan.

So remember, A Tartan is defined by so many yarns of one color, followed by so many yarns of the next color and so on. This is known as the thread count.

Versions of a Tartan are just that, versions. The same Tartan offered in different colors or shades of different colors.