I need a mate to this table. Seen one for sale?  

Are
You sure it didn't have a wood top before it had a travertine top?
Lane marked it underneath, and Lane was very rigorous about marking everything, so if it isn't marked, I am guessing that is because it lost the marked part.
And if I wanted another, I'd get a wood topped one and go down to the local counter top fabricators and get them to make a travertine top for it.

I'm sure. I've looked at the...
I'm sure. I've looked at the wood tables with the same thought as you and the wood tables are not the same underneath the top. The wood tables do not have the cross supports underneath the top so wood can not be removed and travertine thrown on top.
The travertine is not aftermarket. Underneath the travertine there is an x beam with stoppers. The x beam is not glossed, so it wasn't meant to be seen under glass. Can only assume then it was meant to be topped in opaque stone.
The one in the photo is from a store in Long Beach (sold or I wouldn't be asking). My table is in the middle of the country. I've seen a few others on line...all sold. It would be too weird if there were that many alike across the country that were aftermarket.
I know it's odd...it looks so much like the Lane. It's signed in graphite pencil and numbered, but not marked Lane. But the base is too much like the Lane all wood version for it to be different. Thinking that what happened is the Lane label that was branded into their furniture is too large to fit anywhere on this frame base. It would have had to go underneath the table top and since this was travertine, it's one of the few that Lane couldn't brand.

There is one for sale
On ebay right now. Search for "lane walnut travertine"